  We describe a fast implementation of the quasi-centroid molecular dynamics
(QCMD) method in which the quasi-centroid potential of mean force is
approximated as a separable correction to the classical interaction potential.
This correction is obtained by first calculating quasi-centroid radial and
angular distribution functions in a short path integral molecular dynamics
simulation, and then using iterative Boltzmann inversion to obtain an effective
classical potential that reproduces these distribution functions in a classical
NVT simulation. We illustrate this approach with example applications to the
vibrational spectra of gas phase molecules, obtaining excellent agreement with
QCMD reference calculations for water and ammonia and good agreement with the
quantum mechanical vibrational spectrum of methane.
