nbody_Angles.py
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	from nbody_graph_search import Ugraph

	# To find 3-body "angle" interactions, we would use this subgraph:
	#
	#
	# ------* => 1st bond connects atoms 0 and 1
	# 0 1 2 2nd bond connects atoms 1 and 2
	#

	bond_pattern = Ugraph([(0,1), (1,2)])
	# (Ugraph atom indices begin at 0, not 1)


	# The next function eliminates the redundancy between 0-1-2 and 2-1-0:
	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 3-body angle
	interaction between atoms 1, 2, 3 AND 3, 2, 1. This is the same triplet
	of atoms, and the angle between them is the same.)

	"""
	# match[0][0:2] contains the ID numbers for the 3 atoms in the match
	atom0 = match[0][0]
	atom1 = match[0][1]
	atom2 = match[0][2]
	# match[1][0:1] contains the ID numbers for the 2 bonds
	bond0 = match[1][0]
	bond1 = match[1][1]
	if atom0 < atom2:
	#return ((atom0, atom1, atom2), (bond0, bond1)) same thing as:
	return match
	else:
	return ((atom2, atom1, atom0), (bond1, bond0))