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

Graphical abstract: Core- and core-valence correlation contributions are often neglected in studies of larger systems with many cores. The reason is mostly the computational effort, not the insignificance of the neglected effects. The so-called incremental scheme offers a simple way to evaluate these contributions with quantum chemical standard codes. 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. - Abstract: 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 Hg4 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.