Origin of the shoulder peak in sum-frequency vibrational spectroscopy of interfacial water

molecular dynamics simulations and quantum chemistry calculations-which explicitly account for electronic quantum effects-we compute the imaginary part of the SSP SFVS for interfacial water. Our results demonstrate that the shoulder peak of the free OH group stems from both the electric quadrupole and magnetic dipole contributions, with the former being dominant. This approach balances simplicity with physical rigor, potentially offering a new theoretical framework for interpreting complex SFVS.