2006-09-22 00:22:34 +08:00
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<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 hybrid command
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</H3>
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2007-10-03 05:55:02 +08:00
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<H3>pair_style hybrid/overlay command
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</H3>
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2006-09-22 00:22:34 +08:00
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<P><B>Syntax:</B>
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</P>
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<PRE>pair_style hybrid style1 args style2 args ...
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pair_style hybrid/overlay style1 args style2 args ...
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2006-09-22 00:22:34 +08:00
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</PRE>
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<UL><LI>style1,style2 = list of one or more pair styles and their arguments
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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 hybrid lj/cut/coul/cut 10.0 eam lj/cut 5.0
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pair_coeff 1*2 1*2 eam niu3
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pair_coeff 3 3 lj/cut/coul/cut 1.0 1.0
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pair_coeff 1*2 3 lj/cut 0.5 1.2
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</PRE>
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2007-10-03 05:55:02 +08:00
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<PRE>pair_style hybrid/overlay lj/cut 2.5 coul/long 2.0
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pair_coeff * * lj/cut 1.0 1.0
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pair_coeff * * coul/long
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</PRE>
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2006-09-22 00:22:34 +08:00
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<P><B>Description:</B>
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</P>
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2007-10-03 05:55:02 +08:00
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<P>The <I>hybrid</I> and <I>hybrid/overlay</I> styles enable the use of multiple
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pair styles in one simulation. With the <I>hybrid</I> style, exactly one
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pair style is assigned to each pair of atom types. With the
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<I>hybrid/overlay</I> style, one or more pair styles can be assigned to
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each pair of atom types. The assignment of pair styles to type pairs
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is made via the <A HREF = "pair_coeff.html">pair_coeff</A> command.
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</P>
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<P>Here are two examples of hybrid simulations. The <I>hybrid</I> style could
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be used for a simulation of a metal droplet on a LJ surface. The
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metal atoms interact with each other via an <I>eam</I> potential, the
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surface atoms interact with each other via a <I>lj/cut</I> potential, and
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the metal/surface interaction is also computed via a <I>lj/cut</I>
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potential. The <I>hybrid/overlay</I> style could be used as in the 2nd
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example above, where multiple potentials are superposed in an additive
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fashion to compute the interaction between atoms. In this example,
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using <I>lj/cut</I> and <I>coul/long</I> together gives the same result as if
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the <I>lj/cut/coul/long</I> potential were used by itself. In this case,
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it would be more efficient to use the single combined potential, but
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in general any combination of pair potentials can be used together in
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to produce an interaction that is not encoded in any single pair_style
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file, e.g. adding Coulombic forces between granular particles.
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</P>
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<P>All pair styles that will be used are listed as "sub-styles" following
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the <I>hybrid</I> or <I>hybrid/overlay</I> keyword, in any order. Each
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sub-style's name is followed by its usual arguments, as illustrated in
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the example above. See the doc pages of individual pair styles for a
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listing and explanation of the appropriate arguments.
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</P>
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2011-06-02 00:41:07 +08:00
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<P>In the pair_coeff commands, the name of a pair style must be added
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after the I,J type specification, with the remaining coefficients
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being those appropriate to that style. For example, consider a
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simulation with 3 atom types: types 1 and 2 are Ni atoms, type 3 are
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LJ atoms with charges. The following commands would set up a hybrid
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simulation:
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</P>
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2006-09-28 03:12:31 +08:00
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<PRE>pair_style hybrid eam/alloy lj/cut/coul/cut 10.0 lj/cut 8.0
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pair_coeff * * eam/alloy nialhjea Ni Ni NULL
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pair_coeff 3 3 lj/cut/coul/cut 1.0 1.0
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pair_coeff 1*2 3 lj/cut 0.8 1.3
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</PRE>
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2011-06-01 23:21:07 +08:00
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<P>If pair coefficients are specified in the data file read via the
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<A HREF = "read_data.html">read_data</A> command, then the same rule applies.
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E.g. "eam/alloy" or "lj/cut" must be added after the atom type, for
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2011-06-01 23:21:07 +08:00
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each line in the "Pair Coeffs" section, e.g.
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</P>
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2011-06-02 00:41:07 +08:00
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<PRE>Pair Coeffs
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2011-06-01 23:21:07 +08:00
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</PRE>
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2011-06-02 00:41:07 +08:00
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<PRE>1 lj/cut/coul/cut 1.0 1.0
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...
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</PRE>
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<P>Note that the pair_coeff command for some potentials such as
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<A HREF = "pair_eam.html">pair_style eam/alloy</A> includes a mapping specification
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of elements to all atom types, which in the hybrid case, can include
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atom types not assigned to the <I>eam/alloy</I> potential. The NULL
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keyword is used by many such potentials (eam/alloy, Tersoff, AIREBO,
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etc), to denote an atom type that will be assigned to a different
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sub-style.
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</P>
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<P>For the <I>hybrid</I> style, each atom type pair I,J is assigned to exactly
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one sub-style. Just as with a simulation using a single pair style,
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if you specify the same atom type pair in a second pair_coeff command,
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the previous assignment will be overwritten.
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</P>
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<P>For the <I>hybrid/overlay</I> style, each atom type pair I,J can be
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assigned to one or more sub-styles. If you specify the same atom type
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pair in a second pair_coeff command with a new sub-style, then the
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second sub-style is added to the list of potentials that will be
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calculated for two interacting atoms of those types. If you specify
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the same atom type pair in a second pair_coeff command with a
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sub-style that has already been defined for that pair of atoms, then
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the new pair coefficients simply override the previous ones, as in the
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normal usage of the pair_coeff command. E.g. these two sets of
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commands are the same:
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</P>
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<PRE>pair_style lj/cut 2.5
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pair_coeff * * 1.0 1.0
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pair_coeff 2 2 1.5 0.8
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</PRE>
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<PRE>pair_style hybrid/overlay lj/cut 2.5
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pair_coeff * * lj/cut 1.0 1.0
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pair_coeff 2 2 lj/cut 1.5 0.8
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</PRE>
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2009-10-31 06:56:44 +08:00
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<P>Coefficients must be defined for each pair of atoms types via the
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<A HREF = "pair_coeff.html">pair_coeff</A> command as described above, or in the
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data file or restart files read by the <A HREF = "read_data.html">read_data</A> or
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<A HREF = "read_restart.html">read_restart</A> commands, or by mixing as described
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below.
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2007-10-03 05:55:02 +08:00
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</P>
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<P>For both the <I>hybrid</I> and <I>hybrid/overlay</I> styles, every atom type
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pair I,J (where I <= J) must be assigned to at least one sub-style via
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the <A HREF = "pair_coeff.html">pair_coeff</A> command as in the examples above, or
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in the data file read by the <A HREF = "read_data.html">read_data</A>, or by mixing
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as described below.
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</P>
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<P>If you want there to be no interactions between a particular pair of
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atom types, you have 3 choices. You can assign the type pair to some
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sub-style and use the <A HREF = "neigh_modify.html">neigh_modify exclude type</A>
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2007-10-03 05:55:02 +08:00
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command. You can assign it to some sub-style and set the coefficients
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so that there is effectively no interaction (e.g. epsilon = 0.0 in a
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2008-02-20 05:47:37 +08:00
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LJ potential). Or, for <I>hybrid</I> and <I>hybrid/overlay</I> simulations, you
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2011-06-01 23:21:07 +08:00
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can use this form of the pair_coeff command in your input script:
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</P>
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<PRE>pair_coeff 2 3 none
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</PRE>
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<P>or this form in the "Pair Coeffs" section of the data file:
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</P>
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<PRE>3 none
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</PRE>
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2008-02-20 05:47:37 +08:00
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<P>If an assignment to <I>none</I> is made in a simulation with the
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<I>hybrid/overlay</I> pair style, it wipes out all previous assignments of
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that atom type pair to sub-styles.
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</P>
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<P>Note that you may need to use an <A HREF = "atom_style.html">atom_style</A> hybrid
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command in your input script, if atoms in the simulation will need
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attributes from several atom styles, due to using multiple pair
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2006-09-28 03:12:31 +08:00
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potentials.
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2006-09-22 00:22:34 +08:00
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</P>
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2007-06-26 08:03:39 +08:00
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<HR>
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2010-10-26 04:29:55 +08:00
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<P>The potential energy contribution to the overall system due to an
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individual sub-style can be accessed and output via the <A HREF = "compute_pair.html">compute
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pair</A> command.
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</P>
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<HR>
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2010-06-17 23:13:22 +08:00
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<P>IMPORTANT: Several of the potentials defined via the pair_style
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command in LAMMPS are really many-body potentials, such as Tersoff,
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AIREBO, MEAM, ReaxFF, etc. The way to think about using these
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potentials in a hybrid setting is as follows.
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</P>
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<P>A subset of atom types is assigned to the many-body potential with a
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single <A HREF = "pair_coeff.html">pair_coeff</A> command, using "* *" to include
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all types and the NULL keywords described above to exclude specific
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types not assigned to that potential. If types 1,3,4 were assigned in
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that way (but not type 2), this means that all many-body interactions
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between all atoms of types 1,3,4 will be computed by that potential.
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Pair_style hybrid allows interactions between type pairs 2-2, 1-2,
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2-3, 2-4 to be specified for computation by other pair styles. You
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could even add a second interaction for 1-1 to be computed by another
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pair style, assuming pair_style hybrid/overlay is used.
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</P>
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2010-06-29 07:43:03 +08:00
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<P>But you should not, as a general rule, attempt to exclude the
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many-body interactions for some subset of the type pairs within the
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set of 1,3,4 interactions, e.g. exclude 1-1 or 1-3 interactions. That
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is not conceptually well-defined for many-body interactions, since the
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potential will typically calculate energies and foces for small groups
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of atoms, e.g. 3 or 4 atoms, using the neighbor lists of the atoms to
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find the additional atoms in the group. It is typically non-physical
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to think of excluding an interaction between a particular pair of
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atoms when the potential computes 3-body or 4-body interactions.
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</P>
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<P>However, you can still use the pair_coeff none setting or the
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<A HREF = "neigh_modify.html">neigh_modify exclude</A> command to exclude certain
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type pairs from the neighbor list that will be passed to a manybody
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sub-style. This will alter the calculations made by a many-body
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potential, since it builds its list of 3-body, 4-body, etc
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interactions from the pair list. You will need to think carefully as
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to whether it produces a physically meaningful result for your model.
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</P>
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<P>For example, imagine you have two atom types in your model, type 1 for
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atoms in one surface, and type 2 for atoms in the other, and you wish
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to use a Tersoff potential to compute interactions within each
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surface, but not between surfaces. Then either of these two command
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sequences would implement that model:
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</P>
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<PRE>pair_style hybrid tersoff
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pair_coeff * * tersoff SiC.tersoff C C
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pair_coeff 1 2 none
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</PRE>
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<PRE>pair_style tersoff
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pair_coeff * * SiC.tersoff C C
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neigh_modify exclude type 1 2
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</PRE>
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<P>Either way, only neighbor lists with 1-1 or 2-2 interactions would be
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passed to the Tersoff potential, which means it would compute no
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3-body interactions containing both type 1 and 2 atoms.
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2010-06-17 23:13:22 +08:00
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</P>
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<HR>
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2008-01-22 02:12:34 +08:00
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<P><B>Mixing, shift, table, tail correction, restart, rRESPA info</B>:
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2007-06-26 08:03:39 +08:00
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</P>
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<P>Any pair potential settings made via the
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<A HREF = "pair_modify.html">pair_modify</A> command are passed along to all
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sub-styles of the hybrid potential.
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</P>
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<P>For atom type pairs I,J and I != J, if the sub-style assigned to I,I
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and J,J is the same, and if the sub-style allows for mixing, then the
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coefficients for I,J can be mixed. This means you do not have to
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specify a pair_coeff command for I,J since the I,J type pair will be
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assigned automatically to the I,I sub-style and its coefficients
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generated by the mixing rule used by that sub-style. For the
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<I>hybrid/overlay</I> style, there is an additional requirement that both
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the I,I and J,J pairs are assigned to a single sub-style. See the
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"pair_modify" command for details of mixing rules. See the See the
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doc page for the sub-style to see if allows for mixing.
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</P>
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<P>The hybrid pair styles supports the <A HREF = "pair_modify.html">pair_modify</A>
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shift, table, and tail options for an I,J pair interaction, if the
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associated sub-style supports it.
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</P>
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2007-10-03 05:55:02 +08:00
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<P>For the hybrid pair styles, the list of sub-styles and their
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respective settings are written to <A HREF = "restart.html">binary restart
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2010-05-07 23:11:21 +08:00
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files</A>, so a <A HREF = "pair_style.html">pair_style</A> command does
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not need to specified in an input script that reads a restart file.
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However, the coefficient information is not stored in the restart
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file. Thus, pair_coeff commands need to be re-specified in the
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restart input script.
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2007-06-26 08:03:39 +08:00
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</P>
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2007-10-12 07:09:49 +08:00
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<P>These pair styles support the use of the <I>inner</I>, <I>middle</I>, and
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<I>outer</I> keywords of the <A HREF = "run_style.html">run_style respa</A> command, if
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their sub-styles do.
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2007-10-04 08:21:14 +08:00
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</P>
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2007-04-28 00:44:07 +08:00
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<P><B>Restrictions:</B>
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2006-09-22 00:22:34 +08:00
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</P>
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2008-02-20 05:47:37 +08:00
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<P>When using a long-range Coulombic solver (via the
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2010-05-07 23:11:21 +08:00
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<A HREF = "kspace_style.html">kspace_style</A> command) with a hybrid pair_style,
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one or more sub-styles will be of the "long" variety,
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e.g. <I>lj/cut/coul/long</I> or <I>buck/coul/long</I>. You must insure that the
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short-range Coulombic cutoff used by each of these long pair styles is
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the same or else LAMMPS will generate an error.
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2006-09-26 23:30:56 +08:00
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</P>
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2006-09-22 00:22:34 +08:00
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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>
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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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