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 +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.
@@ -86,11 +86,24 @@ 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. +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 ++
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 +
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 @@ -124,6 +137,41 @@ potentials.
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 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 cannot exclude the many-body interactions for some subset of +the type pairs within the full 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. +Additionally it is non-physical to think of excluding an interaction +between a particular pair of atoms when the potential computes 3-body +or 4-body interactions. +
+One exception to this rule is that you can still use the neigh_modify +exclude command to exclude certain type pairs from +the neighbor list that will be passed to each sub-style. This would +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, but +you would need to think carefully as to whether it produces a +physically meaningful result for your model. +
+Mixing, shift, table, tail correction, restart, rRESPA info:
Any pair potential settings made via the diff --git a/doc/pair_hybrid.txt b/doc/pair_hybrid.txt index eea33c7ccd..7f2bf9dd59 100644 --- a/doc/pair_hybrid.txt +++ b/doc/pair_hybrid.txt @@ -72,7 +72,7 @@ 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 such +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. @@ -82,10 +82,23 @@ 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. +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 @@ -120,6 +133,41 @@ potentials. :line +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 cannot exclude the many-body interactions for some subset of +the type pairs within the full 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. +Additionally it is non-physical to think of excluding an interaction +between a particular pair of atoms when the potential computes 3-body +or 4-body interactions. + +One exception to this rule is that you can still use the "neigh_modify +exclude"_neigh_modify.html command to exclude certain type pairs from +the neighbor list that will be passed to each sub-style. This would +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, but +you would need to think carefully as to whether it produces a +physically meaningful result for your model. + +:line + [Mixing, shift, table, tail correction, restart, rRESPA info]: Any pair potential settings made via the