The IR spectrum of liquid water in the OH(D)-stretch region and beyond using q-AQUA-pol and the quantum local monomer theory

This paper reports quantum calculations of the water IR spectrum in the region of the OH(D)-stretch and beyond using the ‘gold-standard’ q-AQUA-pol potential and a previous ab initio dipole moment surface, together with the VSCF/VCI Local Monomer (LMon) quantum method, using the minimum case of three intramolecular and the highest frequency intramolecular mode for each monomer. Var- ious combination bands and the OH(D)-stretch overtone are presented and analysed. These features are a major challenge for widely used classical MD, semi-quantum CMD and RPMD methods, as they struggle to describe overtones and combination bands. The LMon calculations are in good agree- ment with the OH(D)-stretch band both in position and width and capture the higher energy weak overtone and combination bands. For these weak bands, there is good agreement with experiment both in position and intensity and others just with respect to position. These features are analysed in terms of the dominant terms in the VCI expansion of the LMon wavefunction.