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. 2020 Feb 6;16(3):1555–1567. doi: 10.1021/acs.jctc.9b01129

Scheme 1. Three Main Scenarios for the Calculation of Excited States Energies.

Scheme 1

In case I, only the well-separated ground and first excited states are included in the model space. In case II, many states are included in the calculation, but all of them are well separated. In case III, several low-lying excited states are included in the model space, and these are energetically very close to each other. Therefore, their Fock operators will be approximately state-average in contrast to the other cases.