A Fourier Transform Method for Generation of Anharmonic Vibrational Molecular Spectra

Accurate computations of vibrational energies and vibrational spectra of molecules require inclusion of the anharmonic forces. In standard computational protocols, this leads to a large vibrational Hamiltonian matrix that needs to be diagonalized. Spectral intensities are calculated for individual transitions separately. In this work, an alternate direct generation of the spectral curves is proposed, based on a temporal propagation of a trial vibrational wave function followed by the Fourier transformation (FT). The method was applied to model water dimer and fenchone molecules. Arbitrary resolutions could be achieved by longer-time propagations, although a smaller integration time step (∼0.02 fs) was needed for accurate peak frequencies than previously found for similar time-dependent applications within the harmonic approximation. Acceptably accurate relative vibrational spectra intensities were obtained when many random vectors used in the propagations were averaged. For a model fenchone Hamiltonian, simulated Raman and Raman optical activity (ROA) spectral shapes compared well with those obtained by the classical approach. The algorithm is amendable to parallelization. The lack of the lengthy and computer-memory-demanding diagonalization thus makes the FT procedure especially convenient for spectral simulations of larger molecules.