The properties of materials including their interfaces and surfaces can be studied by a variety of advanced experimental techniques. The technique of choice depends on the material and on the property of interest. Likewise, a variety of theoretical/computational methods exist for the study of their ground and excited state properties and the method of choice again depends on the specific problem. In many cases progress in the understanding of the properties has leaped forward thanks to productive interaction between experimentalists and theorists. For such understanding the accurate computational reproduction or prediction of data is necessary but not sufficient, we need also interpretation in terms of (preferably simple) physical/chemical concepts.

This presentation introduces the non-orthogonal configuration interaction (NOCI) method, where the wave function of a molecular electronic state is written as an expansion in terms of a small number of many-electron basis functions (MEBFs),each representing a leading electronic configuration that is expressed in terms of its own, optimized orbitals. The MEBFs are single- or multi-configuration self-consistent field (SCF or MCSCF) wave functions. The orbital sets of different MEBFs are neither identical nor mutually orthogonal and this non-orthogonality complicates the computation of off-diagonal hamiltonian elements in the CI matrix. NOCI can be used to study isolated molecules, but also, when combined with the embedded cluster material model, to describe (rather) localized processes, like core excitations, in materials.

In the past decade we have extended the NOCI method to enable application to ensembles of molecules or fragments: NOCI-F. [1] The MEBFs are then spin-adapted linear combinations of anti-symmetrised products of MCSCF wavefunctions for each molecule/fragment in the ensemble. NOCI-F allows for the study of processes where inter-molecular (or inter-fragment) electron transfer or excitation transfer plays a role.

NOCI and NOCI-F are not computationally simple, but since the final wave functions are short expansions in terms of well-defined molecular states, a clear interpretation in terms of local excitations, charge transfer, etc. can still be given. It is shown how NOCI-F can be used to study multi-exciton generation and magnetic interactions in molecular crystals and electronic excitations involving (molecules on) surfaces.

[1] T. P. Straatsma, R. Broer, A. Sánchez-Mansilla, C. Sousa, and C. de Graaf, GronOR: Scalable and Accelerated Nonorthogonal Configuration Interaction for Molecular Fragment Wave Functions, J. Chem. Theory Comput. 18, 3549–3565 (2022)