Journal Article 4 multireference perturbation theory F2, Be2, and N2 1 Møller-Plesset perturbation theory 1 Rayleigh-Schrödinger perturbation theory 1 correlation energy 1 excited states 1 mehr ... improved virtual orbitals 1 orbital invariance 1 orthogonality constraints 1 potential energy surfaces 1 single reference perturbation theory 1 size extensivity 1 size-extensive formulation 1 spectroscopic constants 1 state-specific MRPT 1 state-specific method 1 unitary group adapted SSMRPT 1 vibrational levels 1 weniger ...