Calculation of anharmonic IR and Raman intensities for periodic systems from DFT Calculations. Implementation and Validation
Résumé
An extension of the CRYSTAL program is presented allowing for calculations of anharmonic Infrared (IR) intensities and Raman activities for periodic systems. This work is a follow up of two papers devoted to the computation of anharmonic vibrational states of solids from DFT calculations, part I: description of the potential energy surface (J. Chem. Theory Comput. 15 (2019) 3755-3765) and part II: implementation of the VSCF and VCI methods (J. Chem. Theory Comput. 15 (2019) 3766-3777). The approach presented here relies on the evaluation of integrals of the dipole moment and polarizability operators over anharmonic wavefunctions obtained from either VSCF or VCI calculations. With this extension, the program now allows for a more complete characterization of the vibrational spectroscopic features of solids within the density functional theory. In particular, it is able (i) to provide reliable positions and inten-sities for most intense spectral features, and (ii) to check whether a first overtone or a combi-nation band has a non-vanishing IR intensity or Raman activity. Therefore, it becomes possi-ble to assign the transition(s) corresponding to satellite peak(s) around a fundamental transi-tion, or the overtones or combination bands that may be as intense as their corresponding fun-damental transitions through the strongest mode-mode couplings, as in so-called Fermi reso-nances. The present method is assessed on two molecular systems, H2O and H2CO, as well as on two solid state cases, Boron hydrides BH4 and their deuterated species BD4 in a crystalline environment of alkali metals (M=Na, K). The solid state cases are particularly insightful as, in the B-H (or B-D) stretching region here considered, they exhibit many spectral features entire-ly due to anharmonic effects: two out of three in the IR spectrum and four out of six in the Raman spectrum. All IR and Raman active overtones and combination bands experimentally observed are correctly predicted with our approach. The effect of the adopted quantum-chemical model (DFT exchange-correlation functional/basis set) for the electronic structure calculations on the computed spectra is discussed and found to be significant, which suggests some special care is needed for the analysis of subtle spectral features.