The Weak Visible n→π* Transition of CF₃NO: A Compact Stress Test for Vertical Excited-State Methods

Trifluoronitrosomethane (CF₃NO) is a small closed-shell molecule that serves as a valuable benchmark for studying low-energy electronic excitations. Although it exhibits only a single weak visible n(N)→π*(N=O) absorption band at 690 nm, its theoretical description proved unexpectedly challenging. This study compared TD-DFT, TDA, EOM-CCSD, and STEOM-DLPNO-CCSD methods using multiple basis sets at the revDSD-PBEP86-D4 optimized geometry. Additional analyses, including CCSD(T) diagnostics, CAS(8,12)/NEVPT2 calculations, spin-orbit coupling evaluation, and ΔSCF density analysis, were performed to investigate possible complications such as static correlation, state mixing, and root assignment errors. Results showed that canonical EOM-CCSD significantly underestimated the excitation wavelength, predicting values near 618 nm, while TDA also produced systematic blue shifts. Diagnostic calculations ruled out strong multireference effects and spin-orbit coupling as major causes. In contrast, TD-DFT/ωB97M-D4 and STEOM-DLPNO-CCSD closely reproduced experimental data, highlighting the importance of response coupling and differential correlation balance.