This defines a transformation of the canonical virtual MOs. For single-excitation wave functions, the matrices ∆P elec and ∆P hole have the same eigenvalues, up to a sign, 99,108 so Λ 2 is the same matrix n occ × n occ in both Eqs. (3.1) and (3.2). (Extra zeros in the latter are needed to dimension the matrices consistently.)