forked from lijiext/lammps
85 lines
2.4 KiB
Plaintext
85 lines
2.4 KiB
Plaintext
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.. index:: compute vacf
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compute vacf command
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====================
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Syntax
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""""""
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.. parsed-literal::
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compute ID group-ID vacf
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* ID, group-ID are documented in :doc:`compute <compute>` command
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* vacf = style name of this compute command
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Examples
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""""""""
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.. parsed-literal::
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compute 1 all vacf
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compute 1 upper vacf
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Description
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"""""""""""
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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 :doc:`variable trap() <variable>` function:
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.. parsed-literal::
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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
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.. note::
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If you want the quantities calculated by this compute to be
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continuous when running from a :doc:`restart file <read_restart>`, 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 :ref:`this section <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 :doc:`units <units>`.
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Restrictions
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""""""""""""
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none
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Related commands
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""""""""""""""""
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:doc:`compute msd <compute_msd>`
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**Default:** none
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.. _lws: http://lammps.sandia.gov
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.. _ld: Manual.html
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.. _lc: Section_commands.html#comm
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