forked from lijiext/lammps
85 lines
2.4 KiB
Plaintext
85 lines
2.4 KiB
Plaintext
.. 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
|