Build a 3-RDM by cumulant expansion and setting 3rd cumulant to zero

d3 approx = D ^ D ^ D + 3 (2C) ^ D

tpdm has normalization (n choose 2) where n is the number of electrons

tpdm np.ndarray

four-tensor representing the two-RDM

n_electrons int

number of electrons in the system

six-tensor reprsenting the three-RDM