def get_angle(self, triplet):
return self._angle_dict.get(triplet)<EOL>
Get a rest angle for a given triplet

           A triplet consists of a tuple with six elements: (n0, v0, n1, v1, n2, v2)
           The indexes refer to consecutive atoms forming a valence angle. n0,
           n1 and n2 are the atom numbers of the angle and v0, v1 and v2 are the
           valences of the corresponding atoms. n1 and v1 are the values for the
           central atom in the angle.