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

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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, De, 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 De, 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.

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