Absolutely localized molecular orbitals (ALMOs) offer a way to construct diabatic states with charge localized on molecular fragments. Electronic structure calculations based on ALMOs can be utilized to perform energy decomposition analysis (EDA) for intermolecular interactions^[1] and to predict electronic couplings between donor and acceptor states in ground-state (thermal) electron transfer processes.^[2] Here, we will show that by combining the variational optimization of ALMOs and the state-targeting ΔSCF approach for electronic excited states, one can construct diabatic states in which electronic excitations are localized molecular fragments. Such an ALMO-ΔSCF approach is fully compatible with the ALMO-EDA framework, enabling the latter to investigate intermolecular interactions involving species in non-Aufbau electronic configurations. In particular, we will show how this new extension of ALMO-EDA can be employed to investigate monovalent alkali earth metal complexes, whose s electron is promoted to one of the p or d orbitals upon the formation of complex. We will also demonstrate how one can use the ALMO-ΔSCF approach to construct a locally excited (LE) state in the scenario of a photoinduced electron transfer process, whose coupling with a charge-transfer (CT) state can be accurately computed using a modified multistate DFT scheme that we developed.^[2,3] Compared to the results of high-level correlated wavefunction methods, our ALMO-based diabatization scheme yields accurate results for excited-state electron and hole transfer that underlie a wide array of important physical processes, ranging from DNA repair to charge separation in photovoltaic materials.^[3] Our development of ALMO-based electronic structure methods thus opens the door to the prediction and interpretation of a broad range of chemical phenomena beyond ground-state electronic configurations, at a computational cost comparable to that of ground-state mean-field DFT calculations.

Thomas Markland

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