Some Useful Odds and Ends From the N-Electron Valence State Perturbation Theory

The n-electron Valence state Perturbation Theory makes use of zero order wavefunctions whose energies are endowed with a direct physical interest, describing various processes occurring in the active space (removal/addition of one or two electrons, electronic excitations). It is shown that the zero order energies related to the process of removal of an electron from the active space provide a reasonable and cheap approximation to the vertical ionization potentials. The zero order energies referring to the process of an electronic excitation within the active space can also provide a first approximation to electronic transition energies, provided that a careful choice of the active molecular orbitals is performed. Test calculations have been carried out on the molecules N2 and H2CO.