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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