Abstract
Antisymmetrized product of strongly orthogonal geminals (APSG) Ansatz is a computationally economic wavefunction class with favorable formal properties. These include extensivity, variational determination of the wavefunction parameters or qualitatively correct description of single bond dissociation. Breaking multiple bonds or non-isolated single bonds results in fragments of incorrect spin state when computed by APSG. This has been identified as a potential problem in APSG-based linearized coupled cluster approach (LCC). An alternative correction scheme based on the extended random phase approximation (ERPA) is investigated from this point of view, in parallel with LCC. The two methods are compared formally. Potential energy curves and atomic spin by APSG-based LCC and ERPA are presented on illustrative examples for multiple bond breaking. Origin of the marked difference between the behavior of LCC and ERPA is explored.
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