forked from lijiext/lammps
143 lines
5.4 KiB
HTML
143 lines
5.4 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>pair_style soft command
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</H3>
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<H3>pair_style soft/gpu command
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</H3>
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<H3>pair_style soft/omp command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_style soft cutoff
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</PRE>
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<UL><LI>cutoff = global cutoff for soft interactions (distance units)
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>pair_style soft 2.5
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pair_coeff * * 10.0
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pair_coeff 1 1 10.0 3.0
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</PRE>
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<PRE>pair_style soft 2.5
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pair_coeff * * 0.0
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variable prefactor equal ramp(0,30)
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fix 1 all adapt 1 pair soft a * * v_prefactor
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>Style <I>soft</I> computes pairwise interactions with the formula
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</P>
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<CENTER><IMG SRC = "Eqs/pair_soft.jpg">
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</CENTER>
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<P>It is useful for pushing apart overlapping atoms, since it does not
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blow up as r goes to 0. A is a pre-factor that can be made to vary in
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time from the start to the end of the run (see discussion below),
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e.g. to start with a very soft potential and slowly harden the
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interactions over time. Rc is the cutoff. See the <A HREF = "fix_nve_limit.html">fix
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nve/limit</A> command for another way to push apart
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overlapping atoms.
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</P>
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<P>The following coefficients must be defined for each pair of atom types
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via the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples above,
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or in the data file or restart files read by the
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<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
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commands, or by mixing as described below:
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</P>
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<UL><LI>A (energy units)
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<LI>cutoff (distance units)
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</UL>
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<P>The last coefficient is optional. If not specified, the global soft
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cutoff is used.
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</P>
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<P>IMPORTANT NOTE: The syntax for <A HREF = "pair_coeff.html">pair_coeff</A> with a
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single A coeff is different in the current version of LAMMPS than in
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older versions which took two values, Astart and Astop, to ramp
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between them. This functionality is now available in a more general
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form through the <A HREF = "fix_adapt.html">fix adapt</A> command, as explained
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below. Note that if you use an old input script and specify Astart
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and Astop without a cutoff, then LAMMPS will interpret that as A and a
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cutoff, which is probabably not what you want.
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</P>
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<P>The <A HREF = "fix_adapt.html">fix adapt</A> command can be used to vary A for one
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or more pair types over the course of a simulation, in which case
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pair_coeff settings for A must still be specified, but will be
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overridden. For example these commands will vary the prefactor A for
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all pairwise interactions from 0.0 at the beginning to 30.0 at the end
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of a run:
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</P>
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<PRE>variable prefactor equal ramp(0,30)
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fix 1 all adapt 1 pair soft a * * v_prefactor
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</PRE>
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<P>Note that a formula defined by an <A HREF = "variable.html">equal-style variable</A>
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can use the current timestep, elapsed time in the current run, elapsed
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time since the beginning of a series of runs, as well as access other
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variables.
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</P>
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<HR>
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<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
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the same as the corresponding style without the suffix. They have
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been optimized to run faster, depending on your available hardware, as
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discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
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manual. The accelerated styles take the same arguments and should
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produce the same results, except for round-off and precision issues.
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</P>
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<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
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packages, respectively. They are only enabled if LAMMPS was built with
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those packages. See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
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section for more info.
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</P>
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<P>You can specify the accelerated styles explicitly in your input script
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by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
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switch</A> when you invoke LAMMPS, or you can
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use the <A HREF = "suffix.html">suffix</A> command in your input script.
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</P>
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<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
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more instructions on how to use the accelerated styles effectively.
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</P>
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<HR>
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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</P>
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<P>For atom type pairs I,J and I != J, the A coefficient and cutoff
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distance for this pair style can be mixed. A is always mixed via a
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<I>geometric</I> rule. The cutoff is mixed according to the pair_modify
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mix value. The default mix value is <I>geometric</I>. See the
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"pair_modify" command for details.
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</P>
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<P>This pair style does not support the <A HREF = "pair_modify.html">pair_modify</A>
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shift option, since the pair interaction goes to 0.0 at the cutoff.
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</P>
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<P>The <A HREF = "pair_modify.html">pair_modify</A> table and tail options are not
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relevant for this pair style.
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</P>
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<P>This pair style writes its information to <A HREF = "restart.html">binary restart
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files</A>, so pair_style and pair_coeff commands do not need
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to be specified in an input script that reads a restart file.
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</P>
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<P>This pair style can only be used via the <I>pair</I> keyword of the
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<A HREF = "run_style.html">run_style respa</A> command. It does not support the
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<I>inner</I>, <I>middle</I>, <I>outer</I> keywords.
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</P>
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<HR>
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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 = "pair_coeff.html">pair_coeff</A>, <A HREF = "fix_nve_limit.html">fix nve/limit</A>, <A HREF = "fix_adapt.html">fix
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adapt</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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