Second-order Møller-Plesset perturbation theory as a configuration and orbital generator in multiconfiguration self-consistent field calculations

It is shown that the magnitude of the natural orbital (NO) occupation numbers of second-order Møller-Plesset (MP2) perturbation theory can be used to select physically reasonable configuration spaces for ground state MCSCF calculations. When the MP2 NO's are used as an initial guess for the orbitals, a second-order Newton-Raphson MCSCF calculation is in the local region from the first iteration. Fast convergence is therefore ensured to a stationary point with orbitals of similar structure as the MP2 NO's, thereby reducing significantly the risk of converging to undesired stationary points.