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Despite its simplicity, the single-trajectory thawed Gaussian approximationhas proven useful for calculating vibrationally resolved electronic spectra ofmolecules with weakly anharmonic potential energy surfaces. Here, we show thatthe thawed Gaussian approximation can capture surprisingly well even moresubtle observables, such as the isotope effects in the absorption spectra, andwe 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 thedifferences in the semiclassical trajectories followed by the fourisotopologues, and the isotope effects-narrowing of the transition band andreduction of the peak spacing-are accurately described by this semiclassicalmethod. In contrast, the adiabatic harmonic model shows a double progressioninstead of the single progression seen in the experimental spectra. Thevertical harmonic model correctly shows only a single progression but fails todescribe the anharmonic peak spacing. Analysis of the normal-mode activationupon excitation provides insight into the elusiveness of the symmetricstretching progression in the spectra.

Isotope effects in the electronic spectra of ammonia from ab initio semiclassical dynamics