Short-time Fourier transform analysis of ab initio molecular dynamics simulation: Collision reaction between NH₄⁺(NH ₃)₂ and NH₃

An analyzing technique of the ab initio molecular dynamics simulation is proposed with the use of short-time Fourier transform (ST-FT). The ST-FT analysis demonstrates the dynamical change of the vibrational states in the simulated system. Numerical assessments are preformed for the collision reaction of the ammonia cluster ion NH₄⁺(NH ₃)₂ with the ammonia monomer NH₃. Spectrogram obtained by the ST-FT method, which corresponds to the time evolution of vibrational power spectra, clarifies the relationship between the vibrational states and the reaction channels such as nonreactive collision, substitution, and incorporation.