Toward anharmonic computations of vibrational spectra for large molecular systems

Abstract

The subtle interplay of several different effects makes the interpretation and analysis of experimental spectra in terms of structural and dynamic characteristics a very challenging task. In this context, theoretical studies can be very helpful, and this is the reason behind the rapid evolution of computational spectroscopy from a highly specialized research field toward a versatile and widespread tool. However, in the case of vibrational spectra of large molecular systems, the most popular approach still relies on a harmonic treatment, because of the difficulty to explore the multidimensional anharmonic potential energy surface. These can be overcome considering that, in many cases, the vibrational transitions are well localized and only some of them are observed experimentally. To this aim, the procedure for the simulation of vibrational spectra of large molecular systems beyond the harmonic approximation is discussed. The quality of system-specific reduced dimensional anharmonic approaches is first validated by comparison with computations taking into account all modes simultaneously for anisole and glycine. Next, the approach is applied to two larger systems, namely glycine adsorbed on a silicon surface and chlorophyll-a in solution, and the results are compared with experimental data showing significant improvement over the standard harmonic approximation. Our results show that properly tailored reduced dimension anharmonic approaches stand as feasible routes for state-of-the-art computational spectroscopy studies and allow to take into account both anharmonic and environmental effects on the spectra even for relatively large molecular systems.

Implementation of a graphical user interface for the virtual multifrequency spectrometer: The VMS‐Draw tool

“…Analysis of anharmonic couplings provides important information on the system under study and can be also used for defining cost-effective reduced-dimensionality anharmonic models. [31,41] An analysis of the cubic force constants allows to improve the choice of the normal modes whose anharmonicity can be safely neglected to reach a given accuracy (see Fig. 12).…”

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

“…The B3LYP/N07D method has been employed to calculate anharmonic frequencies of several closed-and open-shell molecular systems 43,90,117,[125][126][127][128][129][130][131] , and a recent extension of N07D, the SNSD basis set, with the inclusion of diffuse s functions on all atoms and one set of diffuse polarized functions, d on heavy and p on hydrogen atoms, has improved its performance (Double and triple-ζ basis sets of SNS and N07 families, are available for download. 2012; visit http:// dreamslab.sns.it) 132,133 .…”

A new Gaussian MCTDH program: Implementation and validation on the levels of the water and glycine molecules

“…Among other things, this choice allows direct comparison with the second order perturbative approach (VPT2) available in the same code [28][29][30][31] . In particular, we have made use of the double-hybrid B2PLYP functional [32][33][34] and the aug-cc-pVTZ basis set, in view of previous experience about the reliability of this computational model [34][35][36] .…”

“…We have chosen this particular system because its study is a natural continuation of the VPT2 and reduced dimensionality VPT2 calculations previously performed by our group [28][29][30][31] . As mentioned previously, such an approach necessitates the use of low width Gaussian functions in order not to compromise the local harmonic approximation.…”