Nuclear quantum effects in condensed phase: The case of the “association band” of liquid water

The combination band just above 2000 cm-1 appearing in the infrared spectrum of liquid water is well known from experiments, and it is thought to originate from the combination of bending motions of the water molecules and the large-amplitude libration modes of the condensed phase. While classical simulations cannot genuinely reproduce this band, here we show that this exquisitely quantum signal can be simulated by on-the-fly ab initio semiclassical molecular dynamics. The number of atoms involved in reproducing this combination band, which is also known as the "association band" of liquid water, shows that this condensed phase signal is quite local.