  An analytic gradient approach for the computation of derivatives of
parity-violating (PV) potentials with respect to displacements of the nuclei in
chiral molecules is described and implemented within a quasirelativistic
mean-field framework. Calculated PV potential gradients are utilised for
estimating PV frequency splittings between enantiomers in rotational and
vibrational spectra of four chiral polyhalomethanes, i.e. CHBrClF, CHClFI,
CHBrFI and CHAtFI. Values calculated within the single-mode approximation for
the frequency shifts agree well with previously reported theoretical values.
The influence of non-separable anharmonic effects (multi-mode effects) on the
vibrational frequency shifts, which are readily accessible with the present
analytic derivative approach, are estimated for the C--F stretching fundamental
of all four molecules and computed for each of the fundamentals in CHBrClF and
CHAtFI. Multi-mode effects are found to be significant, in particular for the
C--F stretching modes, being for some modes and cases of similar size as the
single-mode contribution.
