Vibrational anharmonicity spectroscopy

Chaban GM, Gerber RB. Anharmonic vibrational spectroscopy of the glycine-water complex calculations for ab initio, empirical and hybrid quantum mechanics/molecular mechanics potentials. J Chem Phys 2001 115 1340-1348. 

First-principles calculations of anharmonic vibrational spectroscopy of large molecules... 

The structure of this review is as follows. In Section 9.2, we briefly discuss methods for computing vibrational states of systems having several coupled vibrational degrees of freedom. This will also cover methods that were not yet adapted for direct use with ab initio potentials, since in our view, such extensions may be possible in the future, at least for some of the algorithms. The focus will be on methods that seem potentially applicable to large polyatomics, rather than those of great accuracy for small systems. Section 9.3 also deals with computational methods for anharmonic vibrational spectroscopy that are applicable to potential surfaces from electronic structure calculations. Our main focus will be on the Vibrational Self-Consistent Field (VSCF) approach in several variants and extensions. The performance of the available method in the present state of the art is discussed in Section 9.4. Future directions are outlined in Section 9.5. 

A variety of methods for performing anharmonic vibrational spectroscopy computations were developed to address these and related systems. At the early stages, essentially all the methods were developed for potential surfaces available as explicit analytic functions of the coordinates. 

In briefly surveying several of the many methods proposed for anharmonic vibrational spectroscopy calculations, we mention also methods used so far only for analytic potential surfaces. For many of these methods, adaptations to algorithms may be feasible, and the methods seem promising in this respect. 

Major progress was made in recent years in ab initio calculations of anharmonic vibrational spectroscopy. One of the important indicators is the good agreement with experiment found in calculations for relatively large molecules, having more than 10 atoms (24 vibrational modes). Treatment of such large systems at a good anharmonic... 

In the spirit of the above observation, anharmonic vibrational spectroscopy calculations provide, by comparison with experiment, an evaluation of the quality of the potential surface used. It seems important to use spectroscopy as a way to compare the relative accuracies of different force fields. The results may, of course, differ from case to case. A crude general picture that seems to emerge from a limited set of small molecules (that includes HjO, HCOOH, CH3COOH) [104] indicates that MP2, B97 and B3LYP are very roughly comparable in the agreement with experimental spectroscopy, while HCTH and BLYP functionals do significantly less well (in this order). Much more can be learned about the quality of potential surfaces from different types of ab initio and DFT methods, but this will require systematic studies for various types of molecules. 

First-principles calculations of anharmonic vibrational spectroscopy of large molecules 187 Table 9.4 OH-stretching overtone excitation frequencies for HNO3... 

The field of Ab initio Anharmonic Vibrational Spectroscopy has seen rapid recent advancements. Already, several methods are at hand, and a variety of codes are available in several of the leading electronic structure suites of programs. It is tempting to speculate on the possible directions of progress in the next few years. 

With regard to the electronic structure methodology, major obstacles must be surmounted before improvements can be made. Calculations with Coupled-Cluster methods, an obvious next step, are far more computationally costly than the presently used MP2, or B3LYP methods. In fact, there are extremely few direct ab initio calculations of anharmonic vibrational spectroscopy at higher than MP2 or DPT levels, even for small polyatomics. From the point of view of ab initio anharmonic spectroscopy, the leap from MP2 to the Coupled-Cluster method seems a bottleneck. One can draw encouragement from faster Coupled-Cluster implementations, so far employed with the perturbation theory anharmonic analysis [116,117]. 

Combined ab initio and anharmonic vibrational spectroscopy calculations for rare gas containing fluorohydrides, HRgF ... 

Anharmonic vibrational spectroscopy calculations for novel rare-gas-containing compounds HXeH, HXeCl, HXeBr, and HXeOH ... 

Experimental and theoretical analysis of thevihrational spectra and theoretical smdy of the structures of 3,6-dichloropyridazine and 3,4,5-trichloropyridazine Anharmonic vibrational spectroscopy of hydrogen-bonded systems directly computed from ab initio potential surfaces (H20) , n = 2, 3 Cl-lHjO), n = 1, 2 H+(H20) , = 1, 2 H2O-CH3OH ... 

Mulliken RS (1952) Molecular compounds and their spectra. J Phys Chem 56 801-822 Kurnig U, Schneider S (1987) Ab initio investigation of the structure of hydrogen haUde-amine complexes in the gas-phase and in a polarizable medium. Int J Quantum Chem 14 47-56 Brindle CA, Chaban GM, Gerber RB et al (2005) Anharmonic vibrational spectroscopy calculations for (NHjKHF) and (NHjXDF). Phys Chem Chem Phys 7 945-954 Leopold KR, Canagaratna M, Phillips JA (1997) Partially bonded molecules from the solid state to the stratosphere. Acc Chem Res 30 57-64... 

Abstract In this chapter we review recent advances in theoretical methods to understand and rationalize anharmonic vibrational spectroscopy (IR-MPD and IR-PD) and collision induced dissociations (CID) in the gas phase. We focused our attention on the application of molecular dynamics-based methods. DFT-based molecular dynamics was shown to be able to reproduce InfraRed Multi-Photon Dissociation (IR-MPD) and InfraRed Pre-Dissociation (IR-PD) action spectroscopy experiments, and help assign the vibrational bands, taking into account finite temperature, conformational dynamics, and various anharmonicities. Crucial examples of dynamical vibrational spectroscopy are given on the protonated AlanH" series (related to IR-MPD in the 800-4,0(X) cm domain), ionic clusters (related to IR-PD in the 3,000-4,(XX) cm region), and neutral peptides (related to IR-MPD in the far-lR). We give examples from simple (e.g., cationized urea) to more complex (e.g., peptides and carbohydrates) molecular systems where molecular dynamics was particularly suited to understanding CID experiments. 

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