Excited state dipole moments from ΔSCF: a benchmark

The molecular electric dipole moment of a given electronic state is a simple indicator for the associated charge distribution, and allows a first assessment of how the molecule is influenced by an oriented external electric field (OEF). If the dipole moments of two energetically close electronic states are significantly different, OEFs can be used to tune the molecular photophysics and photochemistry by modifying the shapes and order of the excited-state potential-energy surfaces. Here, we present a comprehensive benchmark of excited-state dipole moments obtained from ΔSCF methods, which have recently gained renewed attention and offer access to excited-state properties essentially with ground-state technology. We investigate the accuracy of these dipole moments in comparison with TDDFT and wavefunction-based calculations, as well as with literature data. We find that, on average, ΔSCF data do not necessarily improve on TDDFT results, but offer increased accuracy in certain pathological cases. In particular, excited-state dipole moments can be obtained with reasonable accuracy for certain doubly excited states, while these states are not accessible at all for conventional TDDFT calculations. Excited-state dipole moments for charge-transfer states, however, suffer from the DFT overdelocalization error, which can affect a ΔSCF calculation on a charge-separated state more severely than the corresponding TDDFT calculation, since the latter typically starts from a charge-neutral ground-state reference. For push-pull systems like donor-acceptor-substituted polyenes, however, this overdelocalization can lead to beneficial error cancellation with the overestimated charge-transfer observed in the ground state.