Optimized virtual orbital space (OVOS) in coupled-cluster calculations

The optimized virtual orbital space (OVOS) method is described from the perspective of several developments that have occurred since the method was originally introduced 25 years ago during the author's postdoctoral stay in the research group of Rod Bartlett. We describe the OVOS approach in the context of the restricted Hartree–Fock (RHF), unrestricted HF (UHF), and multi-configuration self-consistent field (MCSCF) methods used to generate the reference zero-order wave function for the OVOS calculations.