Pople model chemistry

Finally, and perhaps most important for die vast majority of chemical problems where saturation of the basis set is not a practical possibility, the choice should consider the degree to which other results from that particular basis set at that particular level of theory are available for comparison. For instance, to die extent that there are an enormous number of HF/6-31G(d) results published, and thus a reasonably firm understanding of the specific successes and failures of the model, this can assist in the interpretation of new results - Pople has referred to the collection of all data from a given theoretical prescription as comprising a model chemistry and emphasized the utility of analyzing theoretical performance (and future model development efforts) within such a framework. 

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. 

Petersson, G.A., and Al-Laham, M.A., A complete basis set model chemistry. II. Open-shell systems and the total energies of the first-row atoms, J. Chem. Phys., 94, 6081-6090 (1991). Curtiss, L.A., Raghavachari, K., Redfern, P.C., and Pople, J.A., Gaussian-3 theory using... 

We have performed LSDA and B3LYP calculations, using the 6-311+G(3df) basis set, on the three structures MgNC, MgCN and the transition state that interconnects them (TS). To provide further comparison with conventional ab initio calculations, we also performed MP2 and Complete Basis Set (CBS (48)) calculations. These last are alternative "model chemistries" similar to the G2 or G2(MP2) methods of Pople et al (23). A subset of the results obtained are shown in Tables 2 and 3. 

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... 

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 ... 

Development of model sciences to obtain consistent results without unnecessary computation This approach is illustrated by the Gaussian-2 (G2) model chemistry developed by Pople s group. ... 

First, CASSCF does not satisfy Pople s criteria " for a "model chemistry" a method that can be universally applied in a well-defined standard way and whose performance can thus be assessed by carrying out calculations on a wide range of molecules in a uniform fashion. The problem with CASSCF in this regard is that the choice of MOs for the active space often involves a degree of arbitrariness, and it is not always obvious which orbitals are most important. When symmetry is low or absent, exactly which MOs actually wind... 

Pople and his associates not only wrote the first nonspecialist s ab initio program (Gaussian 70), but also introduced the concept of model chemistries . Numerical computational chemistry attempts to model the real chemistry as closely as possible, but each level of approximation has its own inherent limitations. But such systematic errors tend to cancel when data are compared, e.g., energies by means of isodesmic equations. As these standard theoretical levels become more and more sophisticated, they converge more closely to the real chemistry, but some minor empirical corrections are still employed, e.g., in G2 theory, to improve the results. Experimental approaches also are approximations of the real chemistry discrepancies with theory all too often have revealed the deficiencies of experiment. 

The G2 model chemistry of Pople, et al. provides quite satisfactory energies for most organic molecules. When these energies are available,2 ° they are included in the Table 1, and the energies for some of the charged species and radicals were calculated for this review. The G2 energies include the zero-point energies, and therefore, their correction to 25 °C makes use of the difference between Morr and ZPE. 

Pople J A 1973 Theoretical models for chemistry Energy, Structure, and Reactivity ed D W Smith and W B McRae (New York Wiley) p 51-67... 

These terms were introduced by John Pople, who in 1998 received the Nobel Prize in Chemistry for his work in bringing quantum chemical models into widespread use. 

G3 is a recipe involving a variety of different models with the purpose of providing accurate thermochemical data. Original reference (a) L. A. Curtiss, K. Raghavachari, PC. Redfem, V. Rassolov and J.A. Pople, J. Chem. Phys., 109, 7764 (1998). For an up-to-date, on-line source of G3 data see (b) L A. Curtiss, Computational Thermochemistry, chemistry.anl.gov/compmat/ comptherm.htm ... 

These methods appear rather simple, yet they were the starting point of a long evolution. Gilles Klopman, whose research interests at Case-Western Reserve University later turned to modeling bioactive molecules, was the first to use Sandorfy s methods. Kenichi Fukui made extensive use of them in his well-known work on the structures and reactions of saturated hydrocarbons and their derivatives. Fukui added his frontier orbital considerations. Around 1959 the milieu of developments in quantum chemistry contributed to inspire William N. Lipscomb to conceive the extended Hiickel method, which was subsequently implemented by Lawrence L. Lohr and Roald Hoffmann.83 Soon thereafter, John Pople and his coworkers introduced self-consistent field methods based on the zero-differential overlap approximation.815... 

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