The anharmonic force field of ethylene, C2H4, by means of accurate ab initio calculations

Abstract

The quartic force field of ethylene, C 2 H 4 , has been calculated ab initio using augmented coupled cluster, CCSD͑T͒, methods and correlation consistent basis sets of spdf quality. For the 12 C isotopomers C

Dispersion corrected DFT approaches for anharmonic vibrational frequency calculations: nucleobases and their dimers

“…In order to calculate anharmonic frequencies and IR intensities, the underlying semi-diagonal quartic potential energy and cubic electric dipole moment surfaces have been derived through numerical differentiations on geometries displaced from equilibrium along the normal modes (with a 0.01 Å step). Vibrational wavenumbers have been computed within the generalized VPT2 model (GVPT2), where nearly-resonant contributions are removed from the perturbative treatment (leading to the deperturbed model, DVPT2) and treated in a second step variationally 61,66,68 . This model, as implemented in the GAUSSIAN package 86 , provided accurate vibrational wavenumbers for several semi-rigid systems 31,32,90,[97][98][99]103,104,117,129,130,132,134,135 .…”

“…Such an approach relies on semi-empirical thresholds for Fermi and Darling-Denninson resonances. In the present work, the criteria proposed by Martin et al 68 for Fermi resonances have been chosen as they provide accurate results for fundamental bands 134 , overtones and fundamental transitons 136 . Within GVPT2 computations the force constants related to the strongly anharmonic out-of-plane vibrations of amino group, poorly described by Cartesian normal modes, have been excluded from the vibrational perturbative treatment (SKIPT2 option).…”

“…On the contrary, consistent procedures can be derived from QM computations of vibrational properties beyond the harmonic approximation. Recently, exact solutions for the treatment of few active modes to the vibrational problem for a generic system has been proposed 55 and effective schemes to compute vibrational frequencies within the second order vibrational perturbative (VPT2) [56][57][58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75] or vibrational self-consistent field (VSCF) based [76][77][78][79][80][81][82][83][84][85] approaches have been developed and implemented. In particular, a general VPT2 framework to compute thermodynamic properties, vibrational energies and transition intensities from the vibrational ground state to fundamentals, overtones and combination bands 66,67,[86][87][88] has been developed in our group.…”

First theoretical global line lists of ethylene (12C2H4) spectra for the temperature range 50−700 K in the far-infrared for quantification of absorption and emission in planetary atmospheres

“…The first fourth-order ab initio force-field calculations of ethylene were performed by Martin et al (1995). A series of theoretical studies based on a modified Martin PES were reported by Avila & Carrington Jr. (2011) and by Carter et al (2012b,a), who also published qualitatively correct ab initio intensity predictions for fundamental and lower combination bands.…”

A general time-dependent route to Resonance-Raman spectroscopy including Franck-Condon, Herzberg-Teller and Duschinsky effects

“…In this work, the latter are computed using the second-order vibrational perturbation theory (VPT2) 41,63,64 . The necessary third and semi-diagonal fourth derivatives of the potential energy are obtained by numerical differentiation of the analytic harmonic force constants along the mass-weighted normal coordinates (Q) with the GAUSSIAN default step, 40,41 .…”