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nbody_Dihedrals.py
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Wed, Jul 17, 08:41

nbody_Dihedrals.py
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from nbody_graph_search import Ugraph
# To find 4-body "dihedral" interactions, we would use this subgraph:
#
# 1st bond connects atoms 0 and 1
# *---*---*---* => 2nd bond connects atoms 1 and 2
# 0 1 2 3 3rd bond connects atoms 2 and 3
#
bond_pattern = Ugraph([(0,1), (1,2), (2,3)])
# (Ugraph atom indices begin at 0, not 1)
def canonical_order(match):
"""
When searching for atoms with matching bond patterns GraphMatcher
often returns redundant results. We must define a "canonical_order"
function which sorts the atoms and bonds in a way which is consistent
with the type of N-body interaction being considered.
The atoms (and bonds) in a candidate match are rearranged by the
canonical_order(). Then the re-ordered list of atom and bond ids is
tested against the list of atom/bond ids in the matches-found-so-far,
before it is added to the list of interactions found so far.
(For example, it does not make sense to define a separate 4-body dihedral-
angle interaction between atoms 1, 2, 3, 4 AND 4, 3, 2, 1. The dihedral-
angle is not altered when the order of atoms is reversed, so we arbitrarily
choose whichever order causes the first atom to have the lower atomID.)
"""
# match[0][0:3] contains the ID numbers of the 4 atoms in the match
atom0 = match[0][0]
atom1 = match[0][1]
atom2 = match[0][2]
atom3 = match[0][3]
# match[1][0:2] contains the ID numbers of the the 3 bonds
bond0 = match[1][0]
bond1 = match[1][1]
bond2 = match[1][2]
if atom0 < atom3:
#return ((atom0, atom1, atom2, atom3), (bond0, bond1, bond2)) same as:
return match
else:
return ((atom3, atom2, atom1, atom0), (bond2, bond1, bond0))

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