Local explicit correlation methods

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The aim of the current project is to improve the basis set convergence in local coupled cluster methods by including r12-dependent correlation factors. The methods should be applicable to molecules with 50-100 atoms and yield accurate results with medium-size basis sets. In the first funding period explicitly correlated methods based on closed-shell local M011er-Plesset perturbation theory (LMP2) were successfully developed. These will now be extended to restricted open-shell LMP2 and (approximate) coupled cluster methods. As before, Slater functions (approximated by fixed contractions of Gaussian Geminals) will be used in the correlation factor (F12 methods), but the ansatz will be extended to allow for the use of two or more Slater functions with different exponents. This will provide the additional flexibility that is probably needed to treat core correlation effects or heavier elements. All required integrals will be evaluated using density fitting approximations. The accuracy the new methods will be systematically investigated by computing thermodynamic quantities such as reaction energies, ionization potentials, electron affinities, as well as interaction energies of molecular clusters.