The Phonon Spectrum of Phase-I Ammonia: Reassignment of Lattice Mode Symmetries from Combined Molecular and Lattice Dynamics Calculations

The zone-center phonon spectra of phase-I ammonia and deuterated ammonia have been obtained from plane-wave DFT molecular dynamics and localized basis set harmonic lattice dynamics simulations. These data
have proved to be excellent for benchmarking the two approaches. Significant changes to the assignments of
the experimental low-frequency lattice modes are proposed on the basis of the calculated data. The magnitude
of the splitting of the longitudinal and transverse optical modes has been determined and is shown to be
significant in some cases. The high-frequency internal mode region of the spectrum has also been obtained
and is shown to be in excellent agreement with the results of previous studies. The symmetry coordinates and
Davydov splittings of the internal modes are fully analyzed.