Vibrational spectroscopy of polyatomic materials: Semiempirical calculations of anharmonic couplings and infrared and Raman linewidths in naphthalene and PETN crystals

We have developed a computational approach that yields anharmonic vibrational couplings in molecular crystals. The approach is based on anharmonic vibrational potential-energy surface reconstruction starting from a normal-mode vibrational basis. The method was implemented for semiempirical Hamiltonians with periodic boundary conditions, with applications to crystalline naphthalene and pentaerythritol tetranitrate. For each material, we predicted infrared and Raman linewidths, and vibrational anharmonic couplings associated with upand down-conversions, as well as pure-dephasing processes. Comparison is made to experimental data for Raman linewidths and averaged anharmonic couplings; reasonable agreement is obtained, suggesting that implementation of the method within a first-principles electronic structure framework is warranted.