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Free Computational Chemistry ebook



A Guide to Molecular Mechanics and Quantum Chemical Calculations     by: W.J. Hehre

Wavefunction, Inc 2003 ISBN 1-890661-18-X 812 pages, Hardback Price $75

Professor Warren Hehre is the founder of Wavefunction Inc.

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Preface

Over the span of two decades, molecular modeling has emerged as a viable and powerful approach to chemistry. Molecular mechanics calculations coupled with computer graphics are now widely used in lieu of "tactile models" to visualize molecular shape and quantify steric demands. Quantum chemical calculations, once a mere novelty, continue to play an ever increasing role in chemical research and teaching. They offer the real promise of being able to complement experiment as a means to uncover and explore new chemistry. There are fundamental reasons behind the increased use of calculations, in particular quantum chemical calculations, among chemists. Most important, the theories underlying calculations have now evolved to a stage where a variety of important quantities, among them molecular equilibrium geometry and reaction energetics, may be obtained with sufficient accuracy to actually be of use. Closely related are the spectacular advances in computer hardware over the past decade. Taken together, this means that "good theories" may now be routinely applied to "real systems". Also, computer software has now reached a point where it can be easily used by chemists with little if any special training. Finally, molecular modeling has become a legitimate and indispensable part of the core chemistry curriculum. Just like NMR spectroscopy several decades ago, this will facilitate if not guarantee its widespread use among future generations of chemists. There are, however, significant obstacles in the way of continued progress. For one, the chemist is confronted with too many choices to make, and "too few guidelines" on which to base these choices. The fundamental problem is, of course, that the mathematical equations which arise from the application of quantum mechanics to chemistry and which ultimately govern molecular structure and properties cannot be solved. Approximations need to be made in order to realize equations that can actually be solved. "Severe" approximations may lead to methods which can be widely applied but may not yield accurate information. Less severe approximations may lead to methods which are more accurate but which are too costly to be routinely applied. In short, no one method of calculation is likely to be ideal for all applications, and the ultimate choice of specific methods rests on a balance between accuracy and cost. This guide attempts to help chemists find that proper balance. It focuses on the underpinnings of molecular mechanics and quantum chemical methods, their relationship with "chemical observables", their performance in reproducing known quantities and on the application of practical models to the investigation of molecular structure and stability and chemical reactivity and selectivity.