Quantum-chemical multi-configurational methods yield qualitatively correct wave functions for molecules with close-lying frontier orbitals. This class of methods, however, requires the manual selection of a subset of so-called active orbitals for which all possible configurations are considered. A poor choice of this subset leads to the exclusion of important configurations and therefore to wrong results. Quantitative data further requires perturbation theory to consider the dynamic electron correlation which makes the active orbital selection even more delicate. Currently, these active orbitals are selected based on empirical rules or natural orbital occupation numbers of a precedent calculation. Here, we propose selection criteria that additionally take entropy based orbital entanglement measures into account. We therefore exploit the ability of the density matrix renormalization group (DMRG) algorithm to approximate the entanglement information of a large number of active orbitals. The orbital selection protocol based on these criteria is fully automated and applied for the calculation of the heterolytic dissociation energy of four metallocenes M(Cp)2, with M = V, Mn, Fe and Ni. The final energies are obtained from complete active space self-consistent field calculations with second-order perturbation theory and compared to a highly accurate coupled-cluster approach. We further apply our automated procedure to the calculation of reaction coordinates including several electronically excited states.

Automated selection of active orbital spaces

253rd National Meeting (2017)

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