{'fraction_non_alphanumeric': 0.038288288288288286, 'fraction_numerical': 0.009572072072072071, 'mean_word_length': 5.756653992395437, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Automatic Selection of Active Orbitals from Generalized Valence Bond Orbitals.

The accurate multireference (MR) calculation of a strongly correlated chemical system usually relies on a correct pre-selection of a small number of active orbitals from numerous molecular orbitals (MOs). Currently, the active orbitals are generally determined by using try-and-error method. Such a pre-selection by chemical intuition and personal experience may be tedious or unreliable, especially for large complicated systems, and accordingly the construction of active space becomes a bottleneck for large scale MR calculations. In this work, we propose to automatically select the active orbitals according to the natural orbital occupation numbers (NOONs) by performing black-box generalized valence bond (GVB) calculations. We demonstrate the accuracy of this method through testing calculations of the ground states in various systems, ranging from bond dissociation of diatomic molecules (N2, C2 , Cr2) to conjugated molecules (benzene and hexacene), as well as a binuclear transition metal complex [Mn2O2(H2O)2(terpy)2]3+(terpy = 2,2':6,2''-terpyridine) with active spaces up to (26e, 26o) and comparing with the complete active space self-consistent field (CASSCF), density matrix renormalization group (DMRG)-CASSCF references and other recently proposed inexpensive strategies for constructing active spaces. The results indicate that our method is among the most successful ones within our comparison, providing high-quality initial active orbitals very close to the finally optimized (DMRG-)CASSCF orbitals. The method proposed here is expected to greatly benefit the practical implementation of large active space ground state MR calculations, e.g. large scale DMRG calculations.