Nonorthogonal configuration interaction (NOCI) is an interesting alternative to standard electronic structure methods based on orthogonal orbitals. The method accounts rigorously for orbital relaxation effects and the NOCI-Fragments implementation is among the most versatile methods for the calculation of electronic couplings between fragments that are each described by correlated wave functions. Over the last few years, it has extensively been applied to study the mechanism of singlet fission, but the recent implementation of a general spin coupling scheme for the many-electron basis functions has opened the door to other fields of application such as magnetic interactions and core-level spectroscopy, among others.<br/><br/>Because of these new applications, it has become even more urgent to reduce as much as possible the computational cost of the method. In this contribution we present the implementation of a common molecular orbital (MO) basis in GronOR. Since each electronic state is expressed in its own optimal MOs in NOCI-Fragments, the transformation of the two-electron integrals to this molecular basis does not increase efficiency. Instead, a compact common MO basis is generated by the diagonalisation of the overlap matrix of the different MO sets. The corresponding eigenvectors constitute an over-defined basis set, but eliminating the vectors with the smallest eigenvalues removes the linear dependencies.<br/><br/>The final dimension of the common basis is typically of the order of the number of occupied MOs, depending somewhat on the degree of non-orthogonality of the electronic states under study. Therefore, expressing the two-electron integrals and the non-orthogonal electronic states in this compact common MO basis drastically reduces the number of integrals without loosing accuracy and it makes the method basically independent of the size of the atomic orbital (AO) basis. Examples are given to illustrate the performance improvement of NOCI-Fragments. We also discuss strategies to slow down the steep increase of the number of determinant pairs that need to be considered in the calculation of the NOCI matrix elements upon increasing the active space of the CASSCF fragment wave functions.

GronOR: Reduced common molecular orbital basis for nonorthogonal configuration interaction

ACS Spring Meeting 2021

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