Spin-adapted open-shell state-selective coupled cluster approach and doublet stability of its Hartree–Fock reference

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Abstract
The performance of the unitary group based state‐selective coupled cluster approach, in both linear and quadratic approximations, is examined for the case of doublet ground states of the first two alkaline earth hydrides in the whole range of internuclear separation. It is shown that the doublet instability of the restricted open‐shell Hartree–Fock wave functions is responsible for the singular behavior of the linear coupled cluster potential energy curves, as well as for slight discontinuity in these curves when the bilinear terms are included. The effect of using different types of orbitals is investigated and the results are compared with full or very precise limited configuration interaction results as well as with the coupled cluster results employing the unrestricted Hartree–Fock reference, which is free of the instability problems in open shell systems.

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