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.. index:: compute vacf
compute vacf command
====================
Syntax
""""""
.. parsed-literal::
compute ID group-ID vacf
* ID, group-ID are documented in :doc:`compute <compute>` command
* vacf = style name of this compute command
Examples
""""""""
.. parsed-literal::
compute 1 all vacf
compute 1 upper vacf
Description
"""""""""""
Define a computation that calculates the velocity auto-correlation
function (VACF), averaged over a group of atoms. Each atom's
contribution to the VACF is its current velocity vector dotted into
its initial velocity vector at the time the compute was specified.
A vector of four quantites is calculated by this compute. The first 3
elements of the vector are vx * vx0 (and similarly for the y and z
components), summed and averaged over atoms in the group. Vx is the
current x-component of velocity for the atom, vx0 is the initial
x-component of velocity for the atom. The 4th element of the vector
is the total VACF, i.e. (vx*vx0 + vy*vy0 + vz*vz0), summed and
averaged over atoms in the group.
The integral of the VACF versus time is proportional to the diffusion
coefficient of the diffusing atoms. This can be computed in the
following manner, using the :doc:`variable trap() <variable>` function:
.. parsed-literal::
compute 2 all vacf
fix 5 all vector 1 c_2[4]
variable diff equal dt*trap(f_5)
thermo_style custom step v_diff
.. note::
If you want the quantities calculated by this compute to be
continuous when running from a :doc:`restart file <read_restart>`, then
you should use the same ID for this compute, as in the original run.
This is so that the fix this compute creates to store per-atom
quantities will also have the same ID, and thus be initialized
correctly with time=0 atom velocities from the restart file.
**Output info:**
This compute calculates a global vector of length 4, which can be
accessed by indices 1-4 by any command that uses global vector values
from a compute as input. See :ref:`this section <howto_15>` for an overview of LAMMPS output
options.
The vector values are "intensive". The vector values will be in
velocity^2 :doc:`units <units>`.
Restrictions
""""""""""""
none
Related commands
""""""""""""""""
:doc:`compute msd <compute_msd>`
**Default:** none
.. _lws: http://lammps.sandia.gov
.. _ld: Manual.html
.. _lc: Section_commands.html#comm
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