Characterising a configuration interaction excited state using natural transition geminals

We introduce natural transition geminals as a means to qualitatively understand a transition where double excitations are important. The first two A(1) singlet states of the CH cation are used as an initial example. We calculate these states with configuration interaction singles and state-averaged Monte Carlo configuration interaction (SA-MCCI). For each method, we compare the important natural transition geminals with the dominant natural transition orbitals. We then compare SA-MCCI and full configuration interaction with regards to the natural transition geminals using the beryllium atom. We compare using the natural transition geminals with analysing the important configurations in the CI expansion to give the dominant transition for the beryllium atom and the carbon dimer. Finally, we calculate the natural transition geminals for two electronic excitations of formamide.