  We describe improvements to the quasicentroid molecular dynamics (QCMD)
path-integral method, which was developed recently for computing the infrared
spectra of condensed-phase systems. The main development is an improved
estimator for the intermolecular torque on the quasicentroid. When applied to
qTIP4P/F liquid water and ice, the new estimator is found to remove an
artificial 25 cm$^{-1}$ red shift from the libration bands, to increase
slightly the intensity of the OH stretch band in the liquid, and to reduce
small errors noted previously in the QCMD radial distribution functions. We
also modify the mass-scaling used in the adiabatic QCMD algorithm, which allows
the molecular dynamics timestep to be quadrupled, thus reducing the expense of
a QCMD calculation to twice that of Cartesian centroid molecular dynamics for
qTIP4P/F liquid water at 300 K, and eight times for ice at 150 K.
