Application of the method of asymptotic projection to the calculation of vertical ionization potentials

An alternative method is proposed for the calculation of ionized states corresponding to the excitation of both valence and core electrons. Unlike the well-known approaches, in which such states are constructed by removing an electron from the corresponding spin orbital, the method proposed employs the requirement that all the spin orbitals of the ionized system are orthogonal to the spin orbitals of the neutral system. The proper spin symmetry and the orthogonality of the wave functions of these states to the wave function of the neutral molecule are ensured by a simple-to-use method of taking into account the orthogonality that was proposed by us earlier. The adequacy of the proposed scheme is demonstrated by the calculation of 15 ionization potentials of diatomic and triatomic molecules at different levels of the theory. The results of the calculations, performed in optimized basis sets including three to five Gaussian s functions per electron, are in good agreement with experimental data and state-of-the-art results obtained by other methods in extended basis sets.