Importance of Correctly Describing Charge-Transfer Excitations for Understanding the Chemical Effect in SERS

The enhancement mechanism due to the molecule-surface chemical coupling in surface-enhanced Raman scattering (SERS) is governed to a large extent by the energy difference between the highest occupied molecular orbital (HOMO) of the metal and the lowest unoccupied molecular orbital (LUMO) of the molecule. Here, we investigate the importance of correctly describing charge-transfer excitations, using time-dependent density functional theory (TDDFT), when calculating the chemical coupling in SERS. It is well-known that TDDFT, using traditional functionals, underestimates the position of charge-transfer excitations. Here, we show that this leads to a significant overestimation of the chemical coupling mechanism in SERS. Significantly smaller enhancements are found using long-range corrected (LC) functionals as compared with a traditional generalized gradient approximation (GGA) and hybrid functionals. Enhancement factors are found to be smaller than 530 and typically less than 50. Our results show that it is essential to correctly describe charge-transfer excitations for predicting the chemical enhancement in SERS.