Accurate adiabatic energy surfaces for the ground and first excited states of He +2

Abstract

Different factors affecting the accuracy of the computed energy surfaces of the ground and first excited state of He⁺₂ have been examined, including the choice of the one- and many-particle bases, the configurational space in the MRCI (multi-reference configuration interaction) calculations, and other corrections, such as the Davidson and the full counterpoise (CP) correction. From basis-variation studies, it was concluded that multi-reference direct-CI calculations (MRDCI) using CASSCF MOs and/or natural orbitals (NOs) from a smaller CISD calculation, gave results close to full CI. The computed dissociation energies, D_e, for the ground and first excited state of He⁺₂ were 2.4670 (2.4659) eV and 17.2 (17.1) cm^–1, respectively, at the highest level [without and with CP correction for basis-set superposition errors (BSSE)] of calculation with an [11s8p3d1f] GTO contraction, in reasonably good agreement with previous calculations, and estimated correct values, where available. It is believed that the computed D_e, and the energy surface for the first excited state should be reasonably accurate. However, for the ground state, the effects of multiple f functions and/or functions of higher angular momentum have not been investigated owing to limitation of the available computing resources. This is probably the only weakness in the present study.