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176 lines
7.9 KiB
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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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<H3>pair_style hybrid command
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</H3>
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<H3>pair_style hybrid/overlay 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 hybrid style1 args style2 args ...
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pair_style hybrid/overlay style1 args style2 args ...
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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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<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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<P><B>Description:</B>
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</P>
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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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<P>The pair_coeff commands are also specified exactly as they would be
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for a simulation using only one pair style, with one additional
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argument. Following the I,J type specification, the first argument
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sets the pair sub-style. The remaining arguments are the coefficients
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appropriate to that style. For example, consider a simulation with 3
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atom types: types 1 and 2 are Ni atoms, type 3 are LJ atoms with
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charges. The following commands would set up a hybrid simulation:
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</P>
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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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<P>Note that the pair_coeff command for <I>eam/alloy</I> includes a mapping
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specification of elements to all atom types, even those not assigned
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to the <I>eam/alloy</I> potential. The NULL keyword is used by such
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potentials (eam/alloy, Tersoff, AIREBO, etc), to denote an atom type
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that will be assigned to a different 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. Thus if you specify the same atom
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type pair in a second pair_coeff command, a second sub-style is added
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to the list of potentials that will be calculated for two interactings
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atoms of those types.
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</P>
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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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</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">neigh_modify exclude type</A>
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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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LJ potential). Or, for <I>hybrid</I> and <I>hybrid/overlay</I> simulations, you
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can use this form of the pair_coeff command:
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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>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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potentials.
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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>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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<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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files</A>, so a pair_style command does not need to
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specified in an input script that reads a restart file. However, the
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coefficient information is not stored in the restart file. Thus,
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pair_coeff commands need to be re-specified in the restart input
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script.
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</P>
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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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</P>
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<P><B>Restrictions:</B>
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</P>
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<P>When using a long-range Coulombic solver (via the
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<A HREF = "kspace_style">kspace_style</A> command) with a hybrid pair_style, one or
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more sub-styles will be of the "long" variety, e.g. <I>lj/cut/coul/long</I>
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or <I>buck/coul/long</I>. You must insure that the short-range Coulombic
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cutoff used by each of these long pair styles is the same or else
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LAMMPS will generate an error.
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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>
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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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