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### pair_style hybrid command

### pair_style hybrid/overlay command

Syntax:

    pair_style hybrid style1 args style2 args ...
    pair_style hybrid/overlay style1 args style2 args ...

- style1,style2 = list of one or more pair styles and their arguments

Examples:

    pair_style hybrid lj/cut/coul/cut 10.0 eam lj/cut 5.0
    pair_coeff 1*2 1*2 eam niu3
    pair_coeff 3 3 lj/cut/coul/cut 1.0 1.0
    pair_coeff 1*2 3 lj/cut 0.5 1.2

    pair_style hybrid/overlay lj/cut 2.5 coul/long 2.0
    pair_coeff * * lj/cut 1.0 1.0
    pair_coeff * * coul/long

Description:

The hybrid and hybrid/overlay styles enable the use of multiple pair styles in one simulation. With the hybrid style, exactly one pair style is assigned to each pair of atom types. With the hybrid/overlay style, one or more pair styles can be assigned to each pair of atom types. The assignment of pair styles to type pairs is made via the pair_coeff command.

Here are two examples of hybrid simulations. The hybrid style could be used for a simulation of a metal droplet on a LJ surface. The metal atoms interact with each other via an eam potential, the surface atoms interact with each other via a lj/cut potential, and the metal/surface interaction is also computed via a lj/cut potential. The hybrid/overlay style could be used as in the 2nd example above, where multiple potentials are superposed in an additive fashion to compute the interaction between atoms. In this example, using lj/cut and coul/long together gives the same result as if the lj/cut/coul/long potential were used by itself. In this case, it would be more efficient to use the single combined potential, but in general any combination of pair potentials can be used together in to produce an interaction that is not encoded in any single pair_style file, e.g. adding Coulombic forces between granular particles.

All pair styles that will be used are listed as "sub-styles" following the hybrid or hybrid/overlay keyword, in any order. Each sub-style's name is followed by its usual arguments, as illustrated in the example above. See the doc pages of individual pair styles for a listing and explanation of the appropriate arguments.

The pair_coeff commands are also specified exactly as they would be for a simulation using only one pair style, with one additional argument. Following the I,J type specification, the first argument sets the pair sub-style. The remaining arguments are the coefficients appropriate to that style. For example, consider a simulation with 3 atom types: types 1 and 2 are Ni atoms, type 3 are LJ atoms with charges. The following commands would set up a hybrid simulation:

    pair_style hybrid eam/alloy lj/cut/coul/cut 10.0 lj/cut 8.0
    pair_coeff * * eam/alloy nialhjea Ni Ni NULL
    pair_coeff 3 3 lj/cut/coul/cut 1.0 1.0
    pair_coeff 1*2 3 lj/cut 0.8 1.3

Note that the pair_coeff command for eam/alloy includes a mapping specification of elements to all atom types, even those not assigned to the eam/alloy potential. The NULL keyword is used by such potentials (eam/alloy, Tersoff, AIREBO, etc), to denote an atom type that will be assigned to a different sub-style.

For the hybrid style, each atom type pair I,J is assigned to exactly one sub-style. Just as with a simulation using a single pair style, if you specify the same atom type pair in a second pair_coeff command, the previous assignment will be overwritten.

For the hybrid/overlay style, each atom type pair I,J can be assigned to one or more sub-styles. Thus if you specify the same atom type pair in a second pair_coeff command, a second sub-style is added to the list of potentials that will be calculated for two interactings atoms of those types.

Coefficients must be defined for each pair of atoms types via the pair_coeff command as described above, or in the data file or restart files read by the read_data or read_restart commands, or by mixing as described below.

For both the hybrid and hybrid/overlay styles, every atom type pair I,J (where I <= J) must be assigned to at least one sub-style via the pair_coeff command as in the examples above, or in the data file read by the read_data, or by mixing as described below.

If you want there to be no interactions between a particular pair of atom types, you have 3 choices. You can assign the type pair to some sub-style and use the neigh_modify exclude type command. You can assign it to some sub-style and set the coefficients so that there is effectively no interaction (e.g. epsilon = 0.0 in a LJ potential). Or, for hybrid and hybrid/overlay simulations, you can use this form of the pair_coeff command:

    pair_coeff	2 3 none

If an assignment to none is made in a simulation with the hybrid/overlay pair style, it wipes out all previous assignments of that atom type pair to sub-styles.

Note that you may need to use an atom_style hybrid command in your input script, if atoms in the simulation will need attributes from several atom styles, due to using multiple pair potentials.

Mixing, shift, table, tail correction, restart, rRESPA info:

Any pair potential settings made via the pair_modify command are passed along to all sub-styles of the hybrid potential.

For atom type pairs I,J and I != J, if the sub-style assigned to I,I and J,J is the same, and if the sub-style allows for mixing, then the coefficients for I,J can be mixed. This means you do not have to specify a pair_coeff command for I,J since the I,J type pair will be assigned automatically to the I,I sub-style and its coefficients generated by the mixing rule used by that sub-style. For the hybrid/overlay style, there is an additional requirement that both the I,I and J,J pairs are assigned to a single sub-style. See the "pair_modify" command for details of mixing rules. See the See the doc page for the sub-style to see if allows for mixing.

The hybrid pair styles supports the pair_modify shift, table, and tail options for an I,J pair interaction, if the associated sub-style supports it.

For the hybrid pair styles, the list of sub-styles and their respective settings are written to binary restart files, so a pair_style command does not need to specified in an input script that reads a restart file. However, the coefficient information is not stored in the restart file. Thus, pair_coeff commands need to be re-specified in the restart input script.

These pair styles support the use of the inner, middle, and outer keywords of the run_style respa command, if their sub-styles do.

Restrictions:

When using a long-range Coulombic solver (via the kspace_style command) with a hybrid pair_style, one or more sub-styles will be of the "long" variety, e.g. lj/cut/coul/long or buck/coul/long. You must insure that the short-range Coulombic cutoff used by each of these long pair styles is the same or else LAMMPS will generate an error.

Related commands:

pair_coeff

Default: none