Vibrational Spectra of Molecular Crystals with the Generalized Energy-Based Fragmentation Approach.

The generalized energy-based fragmentation (GEBF) approach for molecular crystals with periodic boundary condition (PBC) (denoted as PBC-GEBF) is extended to allow vibrational spectra of molecular crystals to be easily computed at various theory levels. Within the PBC-GEBF approach, the vibrational frequencies of a molecular crystal can be directly evaluated from molecular quantum chemistry calculations on a series of nonperiodic molecular systems. With this approach, the vibrational spectra of molecular crystals can be calculated with much reduced computational costs at various theory levels, as compared to those required by the methods based on periodic electronic structure theory. By testing the performance of the PBC-GEBF method for two molecular crystals (CO2 and imidazole), we demonstrate that the PBC-GEBF approach can reproduce the results of the methods based on periodic electronic structure theory in predicting vibrational spectra of molecular crystals. We apply the PBC-GEBF method at second-order Møller-Plesset perturbation theory (PBC-GEBF-MP2 in short) to investigate the vibrational spectra of the urea and ammonia borane crystals. Our results show that the PBC-GEBF-MP2 method can provide quite accurate descriptions for the observed vibrational spectra of the two systems under study.