Fully automated incremental evaluation of MP2 and CCSD(T) core, core-valence and valence correlation energies

A recently proposed coupled cluster evaluation of core-core and core-valence correlation effects within the incremental scheme has been extended to perturbative treatments. The accuracy of the approach is demonstrated at the MP2 and CCSD(T) level of theory for various systems from different areas of chemistry, i.e. a binuclear titanium complex, a diallylmagnesium compound, a Hg(4) cluster and various hydration complexes of the sodium cation. Besides the convergence of individual correlation contributions arising from the core and/or valence electron systems the basis set dependence of the contributions was also monitored. Results within chemical accuracy of 1 kcal/mol in the total energies are typically obtained at third order of the incremental expansion. Furthermore a few reasonable simplifications of the incremental core-valence treatment are proposed, which allow a large number of individual calculations to be omitted a priori without a significant loss of accuracy. (C) 2010 Elsevier B.V. All rights reserved