def calc_inertia_tensor(atom_iter):
"""Calculate moment of inertia tensor at the centroid
of the atoms.
"""
al = Structure.AtomList(atom_iter)
centroid = al.calc_centroid()
I = numpy.zeros((3, 3), float)
for atm in al:
x = atm.position - centroid
I[0, 0] += x[1]**2 + x[2]**2
I[1, 1] += x[0]**2 + x[2]**2
I[2, 2] += x[0]**2 + x[1]**2
I[0, 1] += -x[0] * x[1]
I[1, 0] += -x[0] * x[1]
I[0, 2] += -x[0] * x[2]
I[2, 0] += -x[0] * x[2]
I[1, 2] += -x[1] * x[2]
I[2, 1] += -x[1] * x[2]
evals, evecs = numpy.linalg.eig(I)
elist = [(evals[0], evecs[0]), (evals[1], evecs[1]), (evals[2], evecs[2])]
elist.sort()
R = numpy.array((elist[0][1], elist[1][1], elist[2][1]), float)
## make sure the tensor uses a right-handed coordinate system
if numpy.allclose(numpy.linalg.det(R), -1.0):
I = numpy.identity(3, float)
I[0, 0] = -1.0
R = numpy.dot(I, R)
assert numpy.allclose(numpy.linalg.det(R), 1.0)
return centroid, R
