A molecular dynamics study of the far infrared spectrum of liquid water

Abstract

The far infrared spectrum of liquid water at room temperature is calculated by molecular dynamics simulation over the spectral range 0.5–1000 cm−1. It is shown that the experimental absorption intensity can be reproduced satisfactorily provided that the dipole induced dipole mechanism is conveniently implemented in the calculation and the classical profile corrected for quantum effects. The contribution due to exchange overlap dipoles between O and H atoms is also investigated but its role in the genesis of the far infrared (FIR) spectrum is negligible. Although the dipole induced dipole (DID) mechanism is found to be responsible for the peculiar band shape near 200 cm−1 by revealing the intermolecular oscillations of the hydrogen bond network, no other translational band is detected in the region 10–60 cm−1, a result in contradistinction with data put forward recently. Moreover, it is shown that the absorption spectrum is the seat of various cancellation effects between permanent and induced dipoles, effects which are described in detail.