Analytical gradients for unrestricted Hartree–Fock and second order Mo/ller–Plesset perturbation theory with single spin annihilation

Unrestricted Hartree–Fock (UHF) wave functions and Mo/ller–Plesset perturbation theory (UMPn) based on a single spin-unrestricted reference determinant can contain significant contamination from unwanted spin states. This contamination may lead to large distortions of the potential energy surface, particularly at the UMPn level. A simple approximation to projected UMPn theory (PMPn) improves the shape of the potential energy surface significantly. Formulas for analytical gradients of the PUHF and approximate projected UMP2 energies with single annihilation have been derived and programmed. This code has been applied to the optimization of transition states for H+C2H4, H+CH2O and H+C2H2 at the PMP2/6-31G* level.