forked from lijiext/lammps
77 lines
2.6 KiB
HTML
77 lines
2.6 KiB
HTML
<HTML>
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<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>
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<HR>
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<H3>compute vacf command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>compute ID group-ID vacf
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "compute.html">compute</A> command
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<LI>vacf = style name of this compute command
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>compute 1 all vacf
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compute 1 upper vacf
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>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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</P>
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<P>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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</P>
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<P>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 <A HREF = "variable.html">variable trap()</A> function:
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</P>
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<PRE>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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</PRE>
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<P>IMPORTANT NOTE: If you want the quantities calculated by this compute
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to be continuous when running from a <A HREF = "read_restart.html">restart file</A>,
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then you should use the same ID for this compute, as in the original
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run. 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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</P>
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<P><B>Output info:</B>
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</P>
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<P>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 <A HREF = "Section_howto.html#howto_15">this
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section</A> for an overview of LAMMPS output
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options.
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</P>
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<P>The vector values are "intensive". The vector values will be in
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velocity^2 <A HREF = "units.html">units</A>.
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</P>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "compute_msd.html">compute msd</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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