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Yu Liu and Hans-Joachim Werner
Local correlation methods as originally proposed by Pulay have been generalized and implemented for spin-restricted open-shell MP2 (RMP2) and unrestricted coupled cluster (UCCSD) methods, using ROMP2 orbitals. Localization is performed separately in the closed-shell and open-shell orbital spaces. The excitations are restricted to domains, and only strong pairs are treated at the highest level. Provided that the open-shell orbital(s) are well localized, this leads to linear scaling of the computational effort with molecular size. Density fitting approximations are used to compute all integrals. The methods are tested for computing ionization potentials and electron affinities, as well as by studying open-shell copper-oxygen complexes with ammonia ligands. The accuracy is comparable to the corresponding canonical methods. The implementation of perturbative corrections for triple excitations is in progress. |
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