Quantum chemistry methods semi-empirical models

The reactivity of the lubricating oil ZDDP additives was investigated by molecular orbital techniques (Armstrong et al., 1998). Semi-empirical quantum chemistry methods were used to model the structures of some of the complexes... 

A nonlinear 18-parameter model based on 10 molecular descriptors calculated by semi-empirical quantum-chemistry methods, starting from optimized 3D geometries [Bodor et al, 1989 Bodor and Huang, 1992b Huang and Bodor, 1994] ... 

In the seventies, most of the 37 papers (8-24) that we report are quantum chemical calculations, mainly on H502+ (8-14,20) or H30+(14-20) and a few on larger clusters with n=4-6 (8,9). However these last calculations are not accurate, obtained either from semi-empirical methods (8) or with small basis sets (DZ, 4-31G) and at the SCF level in ab initio calculations (9). The first accurate Cl calculations definitely establish the pyramidal geometry of the oxonium ion (15,16). The first ab initio determination of the barrier in H502+ appeared in 1970 (10). An attempt was made to study the effect of Cl on this barrier (11) and the abnormal polarizability of H502+ (12). At the end of this decade appeared the first Cl ab initio calculation on the excited states of H30+ (19) and the first CNDO calculations on excited states of larger clusters (20). In parallel to these quantum chemistry studies, a kinetic model (21) treats large systems with n=20 and 26, a polarisation model (22) is proposed, and a study on the liquid uses a continuum model (23). 

Attempts have been made to calculate the mentioned forces more quantitatively. W. Simon and coworkers195) have been successful in representing their, and other, data by semi-empirical model, so that a reasonable quantitative account of the various reported phenomena can be presented in terms of molecular structure data. To date, little work has been done to apply more rigorous, non-empirical methods of quantum chemistry to problems other than solvation67,157,261,269). 

A general trend which could be noticed over the last few years and which may be expected to develop further in the near future involves a closer coupling between the use of general tools of computational chemistry (ab initio and semi-empirical quantum chemistry, statistical-mechanical simulations) and relaxation theory. When applied to model systems, the computational chemistry methods have the potential of providing new insights on how to develop theoretical models, as well as of yielding estimates of the parameters occurring in the models. 

Many theoretical determinations have been proposed for small clusters. In Monte Carlo calculations, we showed the crucial role of the three-body interactions to describe clusters with N=2 and 3 (59hj). For ab initio quantum chemistry calculations, it is seen that small basis sets systematically overestimate the clustering energies (9, 52). In this case, the Zero-Point Energy correction is large (9), and seems less important with larger basis sets (63b). The role of the correlation contribution is not clear and seems to depend on the clusters considered (52, 63b, 69). The other determinations, from semi-empirical quantum chemical methods (8, 55) or analytical models (22, 50, 67a) are of variable accuracy. 

Using the methods of semi-empirical quantum chemistry can be as instructive as it can be, quite frequently, illusive, when it is disconnected from the real dynamics and the global properties of the interface. The development of theoretical interfacial electrochemistry in the near future will proceed in the direction of a combination of ab initio quantum chemical methods and MD. Progress and obstacles in this direction were recently reviewed (320). MD of the solution part of the interface, based on empirical atom-atom interaction potentials, is often of great help for the verification of global models, rather than direct comparison with experiments. One obvious benefit... 

One of the trends for ealeulation of electronic structure of compound models and radieals and ways of reaction is the quantum-chemical method of calculations. Creation of more perfeet semi-empirical methods in quantum chemistry with applying of eomputers make this trend more long-term in studying of kinetics and mechanisms of different elementary reactions. 

The widely used and ever-improving semi-empirical methods of quantum chemistry provided important results or, at least, prepared the way for a final theoretical treatment of chemical problems through the explicit formulation and modelling in connection with processing the experimental material. The spectrum of data spans from preliminary results to useful trend predictions to first approaches to sets of... 

Combined QM/MM methods open a new era in computational chemistry by making possible the modeling of chemical reaction paths in very large (macroscopic) systems. For the time being, the computational requirements are rather important and many applications use semi-empirical quantum methods to reduce the computation time. In this contribution, we showed that ab initio or DFT approaches are possible too, although they are still rather expensive when many atoms enter the quantum subsystem. 

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