Ab initio theory and calculations of multi-electron excitations

Multi-electron excitations are an important class of electronic transitions in many systems. They are particularly important for modeling the phenomena of multi-exciton generation (MEG), a highly studied and controversial feature of semiconductor nanocrystals. We demonstrate using symmetry adapted cluster theory with configuration interaction (SAC-CI) that multiple excitons (MEs) in PbSe and CdSe nanocrystals can be generated directly upon photoexcitation, and that charging of the nanocrystals dramatically quenches the generation of MEs. The multi-configurational methods for modeling these kinds of excitations are well-established but computationally prohibitive. However, time-dependent Hartree-Fock (TDHF) and time-dependent density functional theory (TDDFT), the standard electronic structure workhorse excitation techniques, are single-configuration methods and have inherent difficulties in capturing states with multi-electron character. In this work, we show that the doubly-excited state is included in the superposition state that can be produced via resonant excitation in real-time TDHF and TDDFT. We then present a multi-reference linear response methodology that describes the superposition state response, including that of the doubly-excited state.