forked from lijiext/lammps
72 lines
2.3 KiB
Plaintext
72 lines
2.3 KiB
Plaintext
"LAMMPS WWW Site"_lws - "LAMMPS Documentation"_ld - "LAMMPS Commands"_lc :c
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:link(lws,http://lammps.sandia.gov)
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:link(ld,Manual.html)
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:link(lc,Section_commands.html#comm)
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:line
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compute vacf command :h3
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[Syntax:]
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compute ID group-ID vacf :pre
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ID, group-ID are documented in "compute"_compute.html command
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vacf = style name of this compute command :ul
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[Examples:]
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compute 1 all vacf
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compute 1 upper vacf :pre
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[Description:]
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Define a computation that calculates the velocity auto-correlation
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function (VACF), averaged over a group of atoms. Each atom's
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contribution to the VACF is its current velocity vector dotted into
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its initial velocity vector at the time the compute was specified.
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A vector of four quantites is calculated by this compute. The first 3
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elements of the vector are vx * vx0 (and similarly for the y and z
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components), summed and averaged over atoms in the group. Vx is the
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current x-component of velocity for the atom, vx0 is the initial
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x-component of velocity for the atom. The 4th element of the vector
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is the total VACF, i.e. (vx*vx0 + vy*vy0 + vz*vz0), summed and
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averaged over atoms in the group.
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The integral of the VACF versus time is proportional to the diffusion
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coefficient of the diffusing atoms. This can be computed in the
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following manner, using the "variable trap()"_variable.html function:
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compute 2 all vacf
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fix 5 all vector 1 c_2\[4\]
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variable diff equal dt*trap(f_5)
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thermo_style custom step v_diff :pre
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NOTE: If you want the quantities calculated by this compute to be
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continuous when running from a "restart file"_read_restart.html, 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-atom
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quantities will also have the same ID, and thus be initialized
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correctly with time=0 atom velocities from the restart file.
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[Output info:]
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This compute calculates a global vector of length 4, which can be
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accessed by indices 1-4 by any command that uses global vector values
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from a compute as input. See "this
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section"_Section_howto.html#howto_15 for an overview of LAMMPS output
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options.
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The vector values are "intensive". The vector values will be in
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velocity^2 "units"_units.html.
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[Restrictions:] none
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[Related commands:]
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"compute msd"_compute_msd.html
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[Default:] none
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