Exploring Electron Affinities, LUMO Energies, and Band Gaps with Electron-Pair Theories.

We introduce the electron attachment equation-of-motion pair coupled cluster doubles (EA-EOM-pCCD) ansatz, which allows us to inexpensively compute electron affinities, energies of unoccupied orbitals, and electron attachment spectra. We assess the accuracy of EA-EOM-pCCD for a representative data set of organic molecules for which experimental data are available, as well as the electron attachment process in uranyl dichloride. EA-EOM-pCCD provides more reliable energies for electron attachment properties than its ionization potential EOM counterpart. The advantage of EA-EOM-pCCD is demonstrated for rylene and rylene diimide units of different chain lengths, where it outperforms the more elaborate EOM-DLPNO-CCSD flavors, reducing errors by an order of magnitude.