Psi4#
Symmetry and the Dimension class#
In Forte, the irreducible representations (irreps) of Abelian point groups are represented using a zero-based integer.
The Cotton ordering of irreps is used, which can be found here.
This ordering is convenient because the direct product of two irreps can be computed using the XOR operator.
For example, consider ${C_2v} symmetry. if ha``=A1 and ``hb , then their direct product can be computed as:
// Assume C2v symmetry
// Cotton ordering: [A1, A2, B1, B2]
int ha = 1; // 1 = 01 = A2
int hb = 3; // 3 = 11 = B2
int hab = ha ^ hb; // 10 = 2 = B1