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rLAMMPS lammps
compute_vacf.html
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<HTML>
<CENTER><A
HREF =
"http://lammps.sandia.gov"
>
LAMMPS WWW Site
</A>
-
<A
HREF =
"Manual.html"
>
LAMMPS Documentation
</A>
-
<A
HREF =
"Section_commands.html#comm"
>
LAMMPS Commands
</A>
</CENTER>
<HR>
<H3>
compute vacf command
</H3>
<P><B>
Syntax:
</B>
</P>
<PRE>
compute ID group-ID vacf
</PRE>
<UL><LI>
ID, group-ID are documented in
<A
HREF =
"compute.html"
>
compute
</A>
command
<LI>
vacf = style name of this compute command
</UL>
<P><B>
Examples:
</B>
</P>
<PRE>
compute 1 all vacf
compute 1 upper vacf
</PRE>
<P><B>
Description:
</B>
</P>
<P>
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.
</P>
<P>
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.
</P>
<P>
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
<A
HREF =
"variable.html"
>
variable trap()
</A>
function:
</P>
<PRE>
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
</PRE>
<P>
IMPORTANT NOTE: If you want the quantities calculated by this compute
to be continuous when running from a
<A
HREF =
"read_restart.html"
>
restart file
</A>
,
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.
</P>
<P><B>
Output info:
</B>
</P>
<P>
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
<A
HREF =
"Section_howto.html#howto_15"
>
this
section
</A>
for an overview of LAMMPS output
options.
</P>
<P>
The vector values are "intensive". The vector values will be in
velocity^2
<A
HREF =
"units.html"
>
units
</A>
.
</P>
<P><B>
Restrictions:
</B>
none
</P>
<P><B>
Related commands:
</B>
</P>
<P><A
HREF =
"compute_msd.html"
>
compute msd
</A>
</P>
<P><B>
Default:
</B>
none
</P>
</HTML>
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