Theoretical chemistry definitions

The term theoretical chemistry may be defined as the mathematical description of chemistry. The term computational chemistry is generally used when a mathematical method is sufficiently well developed that it can be automated for implementation on a computer. Note that the words exact and perfect do not appear in these definitions. Very few aspects of chemistry can be computed exactly, but almost every aspect of chemistry has been described in a qualitative or approximately quantitative computational scheme. The biggest mistake a computational chemist can make is to assume that any computed number is exact. However, just as not all spectra are perfectly resolved, often a qualitative or approximate computation can give useful insight into chemistry if the researcher understands what it does and does not predict. 

In this chapter, we will consider the other half of a model chemistry definition the theoretical method used to model the molecular system. This chapter will serve as an introductory survey of the major classes of electronic structure calculations. The examples and exercises will compare the strengths and weaknesses of various specific methods in more detail. The final section of the chapter considers the CPU, memory and disk resource requirements of the various methods. 

Philosophical" or theoretical chemistry was wide-ranging during most of the nineteenth century. In contrast, late-nineteenth-century physical chemists and twentieth-century physicists tended to narrow the definition of theoretical chemistry, eliminating organic structure theory and making theoretical chemistry almost exclusively physical and mathematical. An early indicator of this trend is Noyes s deletion of structure theory from the course in theoretical chemistry at MIT. A later indicator is the special issue of Chemical Reviews in 1991 which carries the title, "Theoretical Chemistry," and begins with an introductory editorial entitled simply "Quantum Theory of Matter." 5... 

As the definition says, a model is a description of a real phenomenon performed by means of mathematical relationships (Box and Draper, 1987). It follows that a model is not the reality itself it is just a simplified representation of reality. Chemometric models, different from the models developed within other chemical disciplines (such as theoretical chemistry and, more generally, physical chemistry), are characterized by an elevated simplicity grade and, for this reason, their validity is often limited to restricted ranges of the whole experimental domain. 

The following definition of computational chemistry was published in 1985 (6) quantitative modeling of chemical behavior on a computer by the formalisms of theoretical chemistry. Some quantum theoreticians naturally would like to see computational chemistry as a subset of their field (7). However, today the number of scientists employed as computational chemists well exceeds the number employed as theoreticians (8). A recent textbook author (9) views computational chemistry as encompassing not only quantum mechanics, but also molecular mechanics, [energy] minimization, simulations, conformational analysis, and other computer-based methods for understanding and predicting the behavior of molecular systems. ... 

Computational chemistry includes applications of theoretical chemistry, but theoretical chemistry and computational chemistry are definitely not synonymous. Theoretical chemistry involves development of mathematical expressions that model physical reality as such, some of theoretical chemistry entails quantum mechanics (QM). Computational chemistry, on the other hand, involves use of computers on which theoretical and many other algorithms have been programmed. 

Fourcroy introduced a significant innovation in the French didactic tradition by fleshing out the operational foundation of theoretical chemistry. His definition of chemistry as a science which tries to discern the intimate reciprocal action of all the bodies of nature on one another was completely new, indicating his emphasis on operations and affinities rather than on principles. He also discussed analysis and synthesis only as the means of chemistry. True or simple analysis produced unaltered principles false or complicated analysis altered them. Synthesis or combination united different bodies by utilizing a force a tendency that existed between them. Although analysis and synthesis were often found united in the operations of chemistry, true analysis was quite... 

In conclusion, we think that both the researcher s ingenuity and chemical intuition must play a decisive role in the definition of the computational strategy. In our opinion, this is a good conclusion because it shows that the study of reaction mechanisms with theoretical tools is not reduced to a dull repetition of a standard computational procedure, but presents the more challenging aspect of an activity open to an innovative use of the instruments provided by theoretical chemistry. 

As computational chemistry became diversified, so did the role of the computational chemist. In fact, today a good definition of a computational chemist is hard to come by. Protein folding and solvation effects lead many biochemists and biophysicists into computational chemistry. Theoretical chemists probably can still be called theoretical chemists, unless they always did a lot of computer work in the past (e.g., the ab initio-ists), in which case they are now called computational chemists. And all the people who develop the software packages for molecular modeling are computational chemists. These people build in the necessary theoretical chemistry so that a nonspecialist can use the programs. 

There is no quantum-mechanical evidence for spatially directed bonds between the atoms in a molecule. Directed valency is an assumption, made in analogy with the classical definition of molecular frameworks, stabilized by rigid links between atoms. Attempts to rationalize the occurrence of these presumed covalent bonds resulted in the notion of orbital hybridization, probably the single most misleading concept of theoretical chemistry. As chemistry is traditionally introduced at the elementary level by medium of atomic orbitals, chemists are conditioned to equate molecular shape with orbital hybridization, and reluctant to consider alternative models. Here is another attempt to reconsider the issue in balanced perspective. 

An a priori analysis on the reactivity and peculiarities of chemical behavior of molecules is a rather difficult but quite solvable task of theoretical chemistry. If molecules interact with a sohd surface, the complexity of its solution increases repeatedly. This is cause by the circumstances as follows firstly, an interaction occurs between two systems of different nature — molecule and surface that can be considered to be endless at the scale of partner secondly, it is difficult to simulate a surface adequately that is a macrodefect of the crystal periodic structure. Moreover, a definite grade of amorphization of surface layer is a characteristic of even typical crystal [125]. Taking into account probable relaxation and reconstruction of real surface as compared with ideal one, obtaining valid structural information on surface and subsurface layer of solids seems to be rather problematic. A cluster model of solid and its surface that is natural for chemists operating terms of local chemical bonds (despite that it is not quite suitable for the systems with covalent bond) may be considered to be fit for the objects with ionic bonds that are objects of our investigation. 

The general treatment of the theory of chemical reactions presented in this book is based on the usual adiabatic separation of nuclear and electronic motions which permits a definition of the potential energy as a function of internuclear distances This approach proves to be very useful for the study of electronically adiabatic reactions, provided a separation of the rotation of the reacting system, treated as a supermolecule, is possible. In general, such a separation seems to be a bad approximation /10/. A consideration of the coupling of the overall rotation with the internal motions of the system means taking into account the possibility of non-adiabatic transitions from one to another potential energy surface. This is still an unsolved problem of theoretical chemistry which is open for discussion. 

In summary, there is definitely a need for further methodieal developments in the field of NN potentials. There is a huge amount of experienee on NNs in computer science and mathematics, but only a small fraction of this knowledge has been transferred to appKcations in theoretical chemistry so far. This promises further rapid progress in the next years. 

An explicit definition was published in 1985 [A. J. Hopfinger, /. Med. Chem., 28, 1133 (1985)] Quantitative modeling of chemical behavior on a computer by the formalisms of theoretical chemistry. 

Reaction paths are a widely used concept in theoretical chemistry. It is evident that the invariance problem, which was mathematically solved a long time ago (cf. the report given in Ref. [1 ]), penetrates again and again the discussions in this field (see Ref. [2]). We give both the non-invariant and the invariant definitions with respect to the choice of the particular coordinate system for two important kinds of chemical reaction pathways (RP), namely, steepest descent lines (SDP) and gradient extremal (GE) curves. 

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