forked from lijiext/lammps
469 lines
30 KiB
HTML
469 lines
30 KiB
HTML
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<title>compute msd/chunk command — LAMMPS documentation</title>
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<link rel="next" title="compute msd/nongauss command" href="compute_msd_nongauss.html"/>
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<a href="Manual.html" class="icon icon-home"> LAMMPS
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<p class="caption"><span class="caption-text">User Documentation</span></p>
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<ul>
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<li class="toctree-l1"><a class="reference internal" href="Section_intro.html">1. Introduction</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_start.html">2. Getting Started</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_commands.html">3. Commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_packages.html">4. Packages</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_accelerate.html">5. Accelerating LAMMPS performance</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_howto.html">6. How-to discussions</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_example.html">7. Example problems</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_perf.html">8. Performance & scalability</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_tools.html">9. Additional tools</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_modify.html">10. Modifying & extending LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_python.html">11. Python interface to LAMMPS</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_errors.html">12. Errors</a></li>
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<li class="toctree-l1"><a class="reference internal" href="Section_history.html">13. Future and history</a></li>
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</ul>
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<p class="caption"><span class="caption-text">Index</span></p>
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<ul class="current">
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<li class="toctree-l1"><a class="reference internal" href="tutorials.html">Tutorials</a></li>
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<li class="toctree-l1"><a class="reference internal" href="commands.html">Commands</a></li>
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<li class="toctree-l1"><a class="reference internal" href="fixes.html">Fixes</a></li>
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<li class="toctree-l1 current"><a class="reference internal" href="computes.html">Computes</a><ul class="current">
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<li class="toctree-l2"><a class="reference internal" href="compute_ackland_atom.html">compute ackland/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_angle.html">compute angle command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_angle_local.html">compute angle/local command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_angmom_chunk.html">compute angmom/chunk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_basal_atom.html">compute basal/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_body_local.html">compute body/local command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_bond.html">compute bond command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_bond_local.html">compute bond/local command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_centro_atom.html">compute centro/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_chunk_atom.html">compute chunk/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_cluster_atom.html">compute cluster/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_cna_atom.html">compute cna/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_com.html">compute com command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_com_chunk.html">compute com/chunk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_contact_atom.html">compute contact/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_coord_atom.html">compute coord/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_damage_atom.html">compute damage/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_dihedral.html">compute dihedral command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_dihedral_local.html">compute dihedral/local command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_dilatation_atom.html">compute dilatation/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_dipole_chunk.html">compute dipole/chunk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_displace_atom.html">compute displace/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_dpd.html">compute dpd command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_dpd_atom.html">compute dpd/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_erotate_asphere.html">compute erotate/asphere command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_erotate_rigid.html">compute erotate/rigid command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_erotate_sphere.html">compute erotate/sphere command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_erotate_sphere_atom.html">compute erotate/sphere/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_event_displace.html">compute event/displace command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_fep.html">compute fep command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_group_group.html">compute group/group command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_gyration.html">compute gyration command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_gyration_chunk.html">compute gyration/chunk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_heat_flux.html">compute heat/flux command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_hexorder_atom.html">compute hexorder/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_improper.html">compute improper command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_improper_local.html">compute improper/local command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_inertia_chunk.html">compute inertia/chunk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_ke.html">compute ke command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_ke_atom.html">compute ke/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_ke_atom_eff.html">compute ke/atom/eff command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_ke_eff.html">compute ke/eff command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_ke_rigid.html">compute ke/rigid command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_meso_e_atom.html">compute meso/e/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_meso_rho_atom.html">compute meso/rho/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_meso_t_atom.html">compute meso/t/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_msd.html">compute msd command</a></li>
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<li class="toctree-l2 current"><a class="current reference internal" href="#">compute msd/chunk command</a><ul>
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<li class="toctree-l3"><a class="reference internal" href="#syntax">Syntax</a></li>
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<li class="toctree-l3"><a class="reference internal" href="#examples">Examples</a></li>
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<li class="toctree-l3"><a class="reference internal" href="#description">Description</a></li>
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<li class="toctree-l3"><a class="reference internal" href="#restrictions">Restrictions</a></li>
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<li class="toctree-l3"><a class="reference internal" href="#related-commands">Related commands</a></li>
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</ul>
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</li>
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<li class="toctree-l2"><a class="reference internal" href="compute_msd_nongauss.html">compute msd/nongauss command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_omega_chunk.html">compute omega/chunk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_orientorder_atom.html">compute orientorder/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_pair.html">compute pair command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_pair_local.html">compute pair/local command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_pe.html">compute pe command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_pe_atom.html">compute pe/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_plasticity_atom.html">compute plasticity/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_pressure.html">compute pressure command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_property_atom.html">compute property/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_property_chunk.html">compute property/chunk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_property_local.html">compute property/local command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_rdf.html">compute rdf command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_reduce.html">compute reduce command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_reduce.html#compute-reduce-region-command">compute reduce/region command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_rigid_local.html">compute rigid/local command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_saed.html">compute saed command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_slice.html">compute slice command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_contact_radius.html">compute smd/contact/radius command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_damage.html">compute smd/damage command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_hourglass_error.html">compute smd/hourglass/error command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_internal_energy.html">compute smd/internal/energy command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_plastic_strain.html">compute smd/plastic/strain command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_plastic_strain_rate.html">compute smd/plastic/strain/rate command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_rho.html">compute smd/rho command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_tlsph_defgrad.html">compute smd/tlsph/defgrad command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_tlsph_dt.html">compute smd/tlsph/dt command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_tlsph_num_neighs.html">compute smd/tlsph/num/neighs command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_tlsph_shape.html">compute smd/tlsph/shape command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_tlsph_strain.html">compute smd/tlsph/strain command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_tlsph_strain_rate.html">compute smd/tlsph/strain/rate command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_tlsph_stress.html">compute smd/tlsph/stress command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_triangle_mesh_vertices.html">compute smd/triangle/mesh/vertices</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_ulsph_num_neighs.html">compute smd/ulsph/num/neighs command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_ulsph_strain.html">compute smd/ulsph/strain command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_ulsph_strain_rate.html">compute smd/ulsph/strain/rate command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_ulsph_stress.html">compute smd/ulsph/stress command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_smd_vol.html">compute smd/vol command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_sna_atom.html">compute sna/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_sna_atom.html#compute-snad-atom-command">compute snad/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_sna_atom.html#compute-snav-atom-command">compute snav/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_stress_atom.html">compute stress/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_tally.html">compute force/tally command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_tally.html#compute-heat-flux-tally-command">compute heat/flux/tally command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_tally.html#compute-pe-tally-command">compute pe/tally command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_tally.html#compute-pe-mol-tally-command">compute pe/mol/tally command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_tally.html#compute-stress-tally-command">compute stress/tally command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp.html">compute temp command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp.html#compute-temp-kk-command">compute temp/kk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_asphere.html">compute temp/asphere command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_body.html">compute temp/body command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_chunk.html">compute temp/chunk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_com.html">compute temp/com command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_cs.html">compute temp/cs command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_deform.html">compute temp/deform command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_deform_eff.html">compute temp/deform/eff command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_drude.html">compute temp/drude command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_eff.html">compute temp/eff command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_partial.html">compute temp/partial command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_profile.html">compute temp/profile command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_ramp.html">compute temp/ramp command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_region.html">compute temp/region command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_region_eff.html">compute temp/region/eff command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_rotate.html">compute temp/rotate command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_temp_sphere.html">compute temp/sphere command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_ti.html">compute ti command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_torque_chunk.html">compute torque/chunk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_vacf.html">compute vacf command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_vcm_chunk.html">compute vcm/chunk command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_voronoi_atom.html">compute voronoi/atom command</a></li>
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<li class="toctree-l2"><a class="reference internal" href="compute_xrd.html">compute xrd command</a></li>
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</ul>
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</li>
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<li class="toctree-l1"><a class="reference internal" href="pairs.html">Pair Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="bonds.html">Bonds</a></li>
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<li class="toctree-l1"><a class="reference internal" href="angles.html">Angle Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="dihedrals.html">Dihedral Styles</a></li>
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<li class="toctree-l1"><a class="reference internal" href="impropers.html">Improper Styles</a></li>
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</ul>
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<div class="section" id="compute-msd-chunk-command">
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<span id="index-0"></span><h1>compute msd/chunk command</h1>
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<div class="section" id="syntax">
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<h2>Syntax</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="n">ID</span> <span class="n">group</span><span class="o">-</span><span class="n">ID</span> <span class="n">msd</span><span class="o">/</span><span class="n">chunk</span> <span class="n">chunkID</span>
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</pre></div>
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</div>
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<ul class="simple">
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<li>ID, group-ID are documented in <a class="reference internal" href="compute.html"><span class="doc">compute</span></a> command</li>
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<li>msd/chunk = style name of this compute command</li>
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<li>chunkID = ID of <a class="reference internal" href="compute_chunk_atom.html"><span class="doc">compute chunk/atom</span></a> command</li>
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</ul>
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</div>
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<div class="section" id="examples">
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<h2>Examples</h2>
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<div class="highlight-default"><div class="highlight"><pre><span></span><span class="n">compute</span> <span class="mi">1</span> <span class="nb">all</span> <span class="n">msd</span><span class="o">/</span><span class="n">chunk</span> <span class="n">molchunk</span>
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</pre></div>
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</div>
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</div>
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<div class="section" id="description">
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<h2>Description</h2>
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<p>Define a computation that calculates the mean-squared displacement
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(MSD) for multiple chunks of atoms.</p>
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<p>In LAMMPS, chunks are collections of atoms defined by a <a class="reference internal" href="compute_chunk_atom.html"><span class="doc">compute chunk/atom</span></a> command, which assigns each atom
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to a single chunk (or no chunk). The ID for this command is specified
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as chunkID. For example, a single chunk could be the atoms in a
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molecule or atoms in a spatial bin. See the <a class="reference internal" href="compute_chunk_atom.html"><span class="doc">compute chunk/atom</span></a> doc page and <a class="reference internal" href="Section_howto.html#howto-23"><span class="std std-ref">Section 6.23</span></a> for details of how chunks can be
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defined and examples of how they can be used to measure properties of
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a system.</p>
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<p>Four quantites are calculated by this compute for each chunk. The
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first 3 quantities are the squared dx,dy,dz displacements of the
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center-of-mass. The 4th component is the total squared displacement,
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i.e. (dx*dx + dy*dy + dz*dz) of the center-of-mass. These
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calculations include all effects due to atoms passing thru periodic
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boundaries.</p>
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<p>Note that only atoms in the specified group contribute to the
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calculation. The <a class="reference internal" href="compute_chunk_atom.html"><span class="doc">compute chunk/atom</span></a> command
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defines its own group; atoms will have a chunk ID = 0 if they are not
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in that group, signifying they are not assigned to a chunk, and will
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thus also not contribute to this calculation. You can specify the
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“all” group for this command if you simply want to include atoms with
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non-zero chunk IDs.</p>
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<p>The slope of the mean-squared displacement (MSD) versus time is
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proportional to the diffusion coefficient of the diffusing chunks.</p>
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<p>The displacement of the center-of-mass of the chunk is from its
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original center-of-mass position, calculated on the timestep this
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compute command was first invoked.</p>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">The number of chunks <em>Nchunk</em> calculated by the <a class="reference internal" href="compute_chunk_atom.html"><span class="doc">compute chunk/atom</span></a> command must remain constant each
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time this compute is invoked, so that the displacement for each chunk
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from its original position can be computed consistently. If <em>Nchunk</em>
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does not remain constant, an error will be generated. If needed, you
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can enforce a constant <em>Nchunk</em> by using the <em>nchunk once</em> or <em>ids
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once</em> options when specifying the <a class="reference internal" href="compute_chunk_atom.html"><span class="doc">compute chunk/atom</span></a> command.</p>
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</div>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">This compute stores the original position (of the
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center-of-mass) of each chunk. When a displacement is calculated on a
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later timestep, it is assumed that the same atoms are assigned to the
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same chunk ID. However LAMMPS has no simple way to insure this is the
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case, though you can use the <em>ids once</em> option when specifying the
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<a class="reference internal" href="compute_chunk_atom.html"><span class="doc">compute chunk/atom</span></a> command. Note that if
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this is not the case, the MSD calculation does not have a sensible
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meaning.</p>
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</div>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">The initial coordinates of the atoms in each chunk are stored in
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“unwrapped” form, by using the image flags associated with each atom.
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See the <a class="reference internal" href="dump.html"><span class="doc">dump custom</span></a> command for a discussion of
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“unwrapped” coordinates. See the Atoms section of the
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<a class="reference internal" href="read_data.html"><span class="doc">read_data</span></a> command for a discussion of image flags and
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how they are set for each atom. You can reset the image flags
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(e.g. to 0) before invoking this compute by using the <a class="reference internal" href="set.html"><span class="doc">set image</span></a> command.</p>
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</div>
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<div class="admonition note">
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<p class="first admonition-title">Note</p>
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<p class="last">If you want the quantities calculated by this compute to be
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continuous when running from a <a class="reference internal" href="read_restart.html"><span class="doc">restart file</span></a>, then
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you should use the same ID for this compute, as in the original run.
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This is so that the fix this compute creates to store per-chunk
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quantities will also have the same ID, and thus be initialized
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correctly with chunk reference positions from the restart file.</p>
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</div>
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<p>The simplest way to output the results of the compute com/msd
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calculation to a file is to use the <a class="reference internal" href="fix_ave_time.html"><span class="doc">fix ave/time</span></a>
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command, for example:</p>
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<pre class="literal-block">
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compute cc1 all chunk/atom molecule
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compute myChunk all com/msd cc1
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fix 1 all ave/time 100 1 100 c_myChunk[*] file tmp.out mode vector
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</pre>
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<p><strong>Output info:</strong></p>
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<p>This compute calculates a global array where the number of rows = the
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number of chunks <em>Nchunk</em> as calculated by the specified <a class="reference internal" href="compute_chunk_atom.html"><span class="doc">compute chunk/atom</span></a> command. The number of columns =
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4 for dx,dy,dz and the total displacement. These values can be
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accessed by any command that uses global array values from a compute
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as input. See <a class="reference internal" href="Section_howto.html#howto-15"><span class="std std-ref">this section</span></a> for an
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overview of LAMMPS output options.</p>
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<p>The array values are “intensive”. The array values will be in
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distance^2 <a class="reference internal" href="units.html"><span class="doc">units</span></a>.</p>
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</div>
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<div class="section" id="restrictions">
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<h2>Restrictions</h2>
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<blockquote>
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<div>none</div></blockquote>
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</div>
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<div class="section" id="related-commands">
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<h2>Related commands</h2>
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<p><a class="reference internal" href="compute_msd.html"><span class="doc">compute msd</span></a></p>
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<p><strong>Default:</strong> none</p>
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