New methods for strong electron correlation are presented. The basic principles of the multiconfigurational quantum chemical method SplitGAS will be disclosed [Li Manni, J. Chem. Theory Comput., 2013, 9 (8), pp 3375]. The configuration interaction expansion, generated from a generalized active space, GAS, wave function is separated in two parts, a principal part (P) containing the most relevant configurations and an extended part (Q) containing less relevant, but not negligible, configurations. The partition is based on an orbital criterion. L�wdin's partitioning technique is used to map the initial eigenvalue problem to a dimensionality equal to dim(P), whereas a simplified expression is employed for the extended part (Q).This approach has enormous computational advantages, since only the principal space needs to be solved exactly, while the effect of the external space is treated perturbatively. The SplitGAS method has been employed to study the challenging Cr2 with satisfactory results. The method performs better than the complete active space method followed by second-order perturbation theory, CASPT2, in terms of equilibrium bond length, dissociation energy, and vibrational properties. The latest developments of this mehod and its benchamark agaist sets of challenging systems will be reported.A new theory, called Multiconfigurational Pair Density Functional Theory, MC-PDFT, which combines multiconfigurational wavefunctions with density functional theory will also be presented.

New methods for strong correlation and the challenging case of the Cr dimer

248th National Meeting (2014)

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