An accurate theoretical study of strongly correlated systems is often hindered from the size of active space. Currently, the size limit of a CASSCF calculation is about 18 electrons in 18 orbitals. Two approaches that expand the applicability of multi reference methods are the Generalized Active Space SCF1 (GASSCF) and the SplitGAS2 methods. In the GAS approach a number of active spaces are chosen and inter-spaces excitations are controlled by means of constraints on the occupation number. Choice of active spaces and excitations are system dependent. In the SplitGAS method, the GAS CI expansion is partitioned in two parts, the principal, which includes the most important CSFs, and an extended, containing less relevant but not negligible CSFs. An effective Hamiltonian is then generated by means of Löwdin's partitioning technique, with the extended part acting as perturbation to the principal space. A benchmark study for these two methods is presented.3 Different properties are examined, including vertical excitation energies of ozone, furan, pyrrole and free base porphin, the stretching of the O-Ni bond of the linear nickel dioxide and ligand field excitation energies of the [CuCl4]2-. Alternative approaches on the construction of the GASSCF and SplitGAS CI expansion are considered and compared with well-established theoretical methods (CASPT2, MRCI) or experimental data (when available). General guidelines for the optimum applicability of these methods will be provided. 1 D. Ma, G. Li Manni, L. Gagliardi J. Chem. Phys. 2011, 135, 044128. 2 G. Li Manni, D. Ma, F. Aquilante, Francesco, J. Olsen, L. Gagliardi J. Chem. Theory Comput. 2013, 9, 3375. 3 K. D. Vogiatzis, G. Li Manni, S. Stoneburner, D. Ma, L. Gagliardi, in preparation.

Systematic expansion of active spaces beyond the CASSCF limit: A GASSCF/SplitGAS benchmark study

249th National Meeting (2015)

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