7.2.1 Introduction

Q-Chem includes several excited state methods which do not incorporate correlation: CIS, XCIS, and RPA, along with some spin-flip variants thereof. ¹⁸⁸ Casanova D., Krylov A. I.
Phys. Chem. Chem. Phys.
(2020), 22, pp. 4326. Link These methods are sufficiently inexpensive that calculations on large molecules are possible, and are roughly comparable to the HF treatment of the ground state in terms of performance. They tend to yield qualitative rather than quantitative insight. Excitation energies tend to exhibit errors on the order of an electron volt, consistent with the neglect of electron correlation effects, which are generally different in the ground state and the excited state.