Accurate quantum chemistry with spatially varying correlation factors

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High-accuracy first-principles computations in quantum chemistry are able to support research by quantitative predictions. One challenge in such computations is reaching the complete basis set limit of the electronic Schrödinger equation, of which the rate of convergence is particularly governed by the description of short-range electron correlation. F12 theory augments the wavefunction ansatz by correlation factors that explicitly depend on the interelectronic distance and significantly increases the accuracy of the underlying method. One drawback in the established standard procedure in F12 theory is the use of a fixed range parameter in the correlation factor, which cannot be chosen optimal for all purposes, in particular, if a molecule contains atoms with strongly differing valence electron density or if core-valence correlation effects are of importance. In the proposed project, we will introduce a modification of the correlation factor that allows to locally choose the optimal range parameter. At the same time the approach promises to keep nearly all of the advantages of the standard F12 approach. The new ansatz will be implemented and tested, first for correlation treatments using second-order perturbation theory, subsequently also for coupled-cluster theory and multireference approaches. The improved methods will be integrated into high-accuracy protocols targeting in particular compounds with heavier main group elements and transition metals.