Communication: configuration interaction singles has a large systematic bias against charge-transfer states.

We show that standard configuration interaction singles (CIS) has a systematic bias against charge-transfer (CT) states, wherein the computed vertical excitation energies for CT states are disproportionately too high (by >1 eV) as compared with non-CT states. We demonstrate this bias empirically for a set of chemical problems with both inter- and intra-molecular electron transfer, and then, for a small analytical model, we prove that this large difference in accuracy stems from the massive changes in electronic structure that must accompany long-range charge transfer. Thus far, the conclusion from this research is that, even in the context of wave function theory, CIS alone is insufficient for offering a balanced description of excited state surfaces (both CT and non-CT) and explicit electron-electron correlation must be included additionally (e.g., via CIS(D)) for charge-transfer applications.