lammps/doc/pair_hybrid.txt

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pair_style hybrid command :h3
pair_style hybrid/overlay command :h3

[Syntax:]

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

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

[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 :pre

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

[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"_pair_coeff.html 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 :pre

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 many 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.  If you specify the same atom type
pair in a second pair_coeff command with a new sub-style, then the
second sub-style is added to the list of potentials that will be
calculated for two interacting atoms of those types.  If you specify
the same atom type pair in a second pair_coeff command with a
sub-style that has already been defined for that pair of atoms, then
the new pair coefficients simply override the previous ones, as in the
normal usage of the pair_coeff command.  E.g. these two sets of
commands are the same:

pair_style lj/cut 2.5
pair_coeff * * 1.0 1.0
pair_coeff 2 2 1.5 0.8 :pre

pair_style hybrid/overlay lj/cut 2.5
pair_coeff * * lj/cut 1.0 1.0
pair_coeff 2 2 lj/cut 1.5 0.8 :pre

Coefficients must be defined for each pair of atoms types via the
"pair_coeff"_pair_coeff.html command as described above, or in the
data file or restart files read by the "read_data"_read_data.html or
"read_restart"_read_restart.html 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"_pair_coeff.html command as in the examples above, or
in the data file read by the "read_data"_read_data.html, 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"_neigh_modify.html
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 :pre

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"_atom_style.html hybrid
command in your input script, if atoms in the simulation will need
attributes from several atom styles, due to using multiple pair
potentials.

:line

The potential energy contribution to the overall system due to an
individual sub-style can be accessed and output via the "compute
pair"_compute_pair.html command.

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IMPORTANT: Several of the potentials defined via the pair_style
command in LAMMPS are really many-body potentials, such as Tersoff,
AIREBO, MEAM, ReaxFF, etc.  The way to think about using these
potentials in a hybrid setting is as follows.

A subset of atom types is assigned to the many-body potential with a
single "pair_coeff"_pair_coeff.html command, using "* *" to include
all types and the NULL keywords described above to exclude specific
types not assigned to that potential.  If types 1,3,4 were assigned in
that way (but not type 2), this means that all many-body interactions
between all atoms of types 1,3,4 will be computed by that potential.
Pair_style hybrid allows interactions between type pairs 2-2, 1-2,
2-3, 2-4 to be specified for computation by other pair styles.  You
could even add a second interaction for 1-1 to be computed by another
pair style, assuming pair_style hybrid/overlay is used.

But you should not, as a general rule, attempt to exclude the
many-body interactions for some subset of the type pairs within the
set of 1,3,4 interactions, e.g. exclude 1-1 or 1-3 interactions.  That
is not conceptually well-defined for many-body interactions, since the
potential will typically calculate energies and foces for small groups
of atoms, e.g. 3 or 4 atoms, using the neighbor lists of the atoms to
find the additional atoms in the group.  It is typically non-physical
to think of excluding an interaction between a particular pair of
atoms when the potential computes 3-body or 4-body interactions.

However, you can still use the pair_coeff none setting or the
"neigh_modify exclude"_neigh_modify.html command to exclude certain
type pairs from the neighbor list that will be passed to a manybody
sub-style.  This will alter the calculations made by a many-body
potential, since it builds its list of 3-body, 4-body, etc
interactions from the pair list.  You will need to think carefully as
to whether it produces a physically meaningful result for your model.

For example, imagine you have two atom types in your model, type 1 for
atoms in one surface, and type 2 for atoms in the other, and you wish
to use a Tersoff potential to compute interactions within each
surface, but not between surfaces.  Then either of these two command
sequences would implement that model:

pair_style hybrid tersoff
pair_coeff * * tersoff SiC.tersoff C C
pair_coeff 1 2 none :pre

pair_style tersoff
pair_coeff * * SiC.tersoff C C
neigh_modify exclude type 1 2 :pre

Either way, only neighbor lists with 1-1 or 2-2 interactions would be
passed to the Tersoff potential, which means it would compute no
3-body interactions containing both type 1 and 2 atoms.

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

Any pair potential settings made via the
"pair_modify"_pair_modify.html 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"_pair_modify.html
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"_restart.html, so a "pair_style"_pair_style.html 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"_run_style.html command, if
their sub-styles do.

[Restrictions:]

When using a long-range Coulombic solver (via the
"kspace_style"_kspace_style.html 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"_pair_coeff.html

[Default:] none