The unrestricted natural orbital–complete active space (UNO–CAS) method: An inexpensive alternative to the complete active space–self-consistent-field (CAS–SCF) method

Abstract

Full configuration interaction in the space of fractionally occupied unrestricted Hartree–Fock natural orbitals is proposed as an inexpensive alternative to the complete active space–self-consistent-field method for ground states. In most cases, this method requires only a fraction of the computational effort of the latter and gives comparable results, as demonstrated on several examples. Gradient evaluation, while more complex in formulation, is computationally not significantly more expensive than unrestricted Hartree–Fock gradients.