Theoretical model chemistry

The implementation of such a theoretical model is termed a theoretical-model chemistry, or simply a model chemistry. 

Gaussian includes many different model chemistries. The theoretical model chemistries in Gaussian have been subjected to the testing procedure described previously and so may be recommended for general use with any system for which they are computationally feasible. 

Model chemistries are characterized by the combination of theoretical procedure and basis set. Every calculation performed with Gaussian must specify the desired theoretical model chemistry in addition to specifying the molecular system to consider and which results to compute for it. 

Theoretical Model Chemistry. The set of results following from application of a particular Theoretical Model. 

THEORETICAL MODEL CHEMISTRY AND ITS RELEVANCE TO THE OPEN-SHELL FORMALISMS... 

A Theoretical Model Chemistry /18,24/ has an underlying approximation scheme which is of potentially uniform precision for all the phenomena it wants to describe, so that the viability of the model can be tested by an appeal to the experimental results. The criteria that a theoretical model should satisfy are the following ... 

J The plural is used here since different theoretical model chemistries give rise to different spectrometers . In rough terms, the methods and approximations which constitute different theoretical model chemistries can be liken to the different component parts of a spectrometer. 

We begin this section by considering the concept of a theoretical model chemistry and the development of a numerical spectrometer in section 1. In section 2, we briefly consider the complementary probes of matter which are used in modern science to elucidate the structure and properties of matter on an atomic scale. In section 3, we discuss how the very concept of atoms and molecules depends on the probes employed and how in modern science, computers provide one of the most powerful probes. The complementarity of different probes of matter is briefly described in section 4. The different perspectives given by complementary probes are emphasised and the greater potency acquired when probes are used in conjunction in a problem based environment is underlined. 

This approach results in what Pople and his co-workers term a "theoretical model chemistry . Systematic comparison of the results obtained by application of this model with corresponding experimental data allows the model to acquire some predictive capability in situations where experiment is either difficult or impossible, or simply too expensive. 

Before delving into the techniques, a semantic excursion seems necessary. First, computational quantum chemistry as used in this chapter reflects the broader definition, referring to any technique that uses computers to model a chemical system via the Schrodinger equation or some approximation thereof this is a catch-all for every ab initio method, semiempirical scheme, and theoretical model chemistry. (Density functional theory also is included, although it does not stringently satisfy this definition, because it enjoys widespread identification with the ab initio methods.) Molecular mechanics, therefore, is not computational quantum chemistry, but its application to hybrid QM/MM methods will be discussed regardless. 

In 1976, Pople, Binkley and Seeger63 listed the properties that ideally one would like to ascribe to a theoretical model chemistry . Almost a quarter of a... 

Hence a revised list of properties that one might wish for a theoretical model chemistry to have reads as follows ... 

This second approach leads to what Pople and his co-workers term a theoretical model chemistry. In practical applications, the complete basis set limit for full configuration interaction cannot be achieved with finite computing resources, except for the very smallest systems. Compromises have to be made in order to achieve a wide range of applicability. Geometry optimization may, for example, be carried out with some lower level theory and/or basis set of moderate size followed by more accurate calculations using higher level theory and/or an extended ... 

AH practical applications of the methods of molecular quantum mechanics involve approximations. They involve the construction of what Pople and his coworkers have termed a theoretical model chemistry. I concluded my last review with a hst of properties that one might wish a theoretical model chemistry to have. It read as follows ... 

While the general strategy of dividing electron correlations into nondynamical and dynamical effects is similar to that of multireference methods (1) such as complete active space (CAS) methods (2,5), there is a crucial difference. Specifically, our insistence on the use of the full valence or perfect pairing active space means that there is no need to customize the number of active orbitals for each problem. In other words, techniques based on this procedure will meet the requirements of a theoretical model chemistry. Of course this means that much larger active spaces will be generated for a given... 

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