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git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@4361 f3b2605a-c512-4ea7-a41b-209d697bcdaa

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sjplimp 2010-06-28 23:43:03 +00:00
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49
doc/pair_hybrid.html
View File

@ -153,22 +153,41 @@ Pair_style hybrid allows interactions between type pairs 2-2, 1-2,
could even add a second interaction for 1-1 to be computed by another
pair style, assuming pair_style hybrid/overlay is used.
</P>
<P>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.
<P>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.
</P>
<P>One exception to this rule is that you can still use the <A HREF = "neigh_modify.html">neigh_modify
exclude</A> 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.
<P>However, you can still use the pair_coeff none setting or the
<A HREF = "neigh_modify.html">neigh_modify exclude</A> 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.
</P>
<P>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:
</P>
<PRE>pair_style hybrid tersoff
pair_coeff * * tersoff SiC.tersoff C C
pair_coeff 1 2 none
</PRE>
<PRE>pair_style tersoff
pair_coeff * * SiC.tersoff C C
neigh_modify exclude type 1 2
</PRE>
<P>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.
</P>
<HR>

49
doc/pair_hybrid.txt
View File

@ -149,22 +149,41 @@ Pair_style hybrid allows interactions between type pairs 2-2, 1-2,
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.
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.
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.
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.
:line