Understanding vibrational anharmonicity and phonon dispersion in solid ammonia borane

We compute the hydrogen vibrational spectra for bulk ammonia borane (NH ₃BH ₃), for both protonated and deuterated cases, using harmonic and anharmonic methodologies as well as low frequency coupling and quantifying dispersive effects to understand their influence on the resulting spectra. Even at 10 K the accounting of anharmonic effects on the incoherent inelastic neutron scattering (IINS) signal is more significant than approximations introduced by sampling and interpolating a finite simulation cell to capture dispersion effects. We compare our computed anharmonic spectrum with IINS measurements and find excellent agreement.