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.