  We assess the cavity molecular dynamics method for the calculation of
vibrational polariton spectra, using liquid water as a specific example. We
begin by disputing a recent suggestion that nuclear quantum effects may lead to
a broadening of polariton bands, finding instead that they merely result in
anharmonic red shifts in the polariton frequencies. We go on to show that our
simulated cavity spectra can be reproduced to graphical accuracy with a
harmonic model that uses just the cavity-free spectrum and the geometry of the
cavity as input. We end by showing that this harmonic model can be combined
with the experimental cavity-free spectrum to give results in good agreement
with optical cavity measurements. Since the input to our harmonic model is
equivalent to the input to the transfer matrix method of applied optics, we
conclude that cavity molecular dynamics cannot provide any more insight into
the effect of vibrational strong coupling on the absorption spectrum than this
transfer matrix method, which is already widely used by experimentalists to
corroborate their cavity results.

Chemical Physics;Quantum Physics