  Despite its simplicity, the single-trajectory thawed Gaussian approximation
has proven useful for calculating vibrationally resolved electronic spectra of
molecules with weakly anharmonic potential energy surfaces. Here, we show that
the thawed Gaussian approximation can capture surprisingly well even more
subtle observables, such as the isotope effects in the absorption spectra, and
we demonstrate it on the four isotopologues of ammonia (NH$_{3}$, NDH$_{2}$,
ND$_{2}$H, ND$_{3}$). The differences in their computed spectra are due to the
differences in the semiclassical trajectories followed by the four
isotopologues, and the isotope effects-narrowing of the transition band and
reduction of the peak spacing-are accurately described by this semiclassical
method. In contrast, the adiabatic harmonic model shows a double progression
instead of the single progression seen in the experimental spectra. The
vertical harmonic model correctly shows only a single progression but fails to
describe the anharmonic peak spacing. Analysis of the normal-mode activation
upon excitation provides insight into the elusiveness of the symmetric
stretching progression in the spectra.
