title: Analytic calculations of anharmonic infrared and Raman vibrational spectra.
abstract: Using a recently developed recursive scheme for the calculation of high-order geometric derivatives of frequency-dependent molecular properties [Ringholm et al., J. Comp. Chem., 2014, 35, 622], we present the first analytic calculations of anharmonic infrared (IR) and Raman spectra including anharmonicity both in the vibrational frequencies and in the IR and Raman intensities. In the case of anharmonic corrections to the Raman intensities, this involves the calculation of fifth-order energy derivatives-that is, the third-order geometric derivatives of the frequency-dependent polarizability. The approach is applicable to both Hartree-Fock and Kohn-Sham density functional theory. Using generalized vibrational perturbation theory to second order, we have calculated the anharmonic infrared and Raman spectra of the non- and partially deuterated isotopomers of nitromethane, where the inclusion of anharmonic effects introduces combination and overtone bands that are observed in the experimental spectra. For the major features of the spectra, the inclusion of anharmonicities in the calculation of the vibrational frequencies is more important than anharmonic effects in the calculated infrared and Raman intensities. Using methanimine as a trial system, we demonstrate that the analytic approach avoids errors in the calculated spectra that may arise if numerical differentiation schemes are used.
authors.affiliation: Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Tromsø-The Arctic University of Norway, N-9037 Tromsø, Norway. yann.cornaton@uit.no magnus.ringholm@uit.no.
authors.forename: Yann
authors.initials: Y
authors.lastname: Cornaton
authors.affiliation: Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Tromsø-The Arctic University of Norway, N-9037 Tromsø, Norway. yann.cornaton@uit.no magnus.ringholm@uit.no.
authors.forename: Magnus
authors.initials: M
authors.lastname: Ringholm
authors.affiliation: Laboratory of Theoretical Chemistry, UCPTS, University of Namur, Rue de Bruxelles 61, B-5000 Namur, Belgium.
authors.forename: Orian
authors.initials: O
authors.lastname: Louant
authors.affiliation: Centre for Theoretical and Computational Chemistry, Department of Chemistry, University of Tromsø-The Arctic University of Norway, N-9037 Tromsø, Norway. yann.cornaton@uit.no magnus.ringholm@uit.no.
authors.forename: Kenneth
authors.initials: K
authors.lastname: Ruud