Open-shell M∅ller—Plesset perturbation theory

In a previous paper, the spin constrained unrestricted Hartree—Fock method (SUHF) was introduced, in which the UHF method was amended by a constraint that 〈Ŝ2〉 should have a prescribed value with λ the associated Lagrange multiplier. It was shown that the limit λ→∞ gave the high spin restricted open-shell Hartree—Fock (ROHF) wavefunction and energy, although the orbitals are rotated. Here it is shown how the λ→∞ results are achieved analytically directly from ROHF calculations. M∅ller—Plesset perturbation theory may then be set up within the amended Fock operators of SUHF theory, based upon the unrestricted formalism. Single replacement contributions enter into the first-order wavefunction. The convergence of this restricted open-shell M∅ller—Plesset perturbation theory (ROMP) is examined to high order using our full configuration interaction program. The calculations show none of the slow convergence properties associated with the UMP series. For NH2 (C2v, 1.5re) and CN (re). ROMP2 and ROMP4 are a substantial improvement over UMP2 and UMP4.