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Thermochemistry in Gaussian
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Joseph W. Ochterski, Ph.D. help@gaussian.com c 2000, Gaussian, Inc. June 2, 2000 The purpose of this paper is to explain how various thermochemical values are computed in Gaussian. The paper documents what equations are used to calculate the quantities, but doesn't explain them in great detail, so a basic understanding of statistical mechanics concepts, such as partition functions, is assumed. Gaussian thermochemistry output is explained, and a couple of examples, including calculating the enthalpy and Gibbs free energy for a reaction, the heat of formation of a molecule and absolute rates of reaction are worked out. Contents 1 Introduction 2 2 Sources of components for thermodynamic quantities 2 2.1 Contributions from translation . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2 Contributions from electronic motion . . . . . . . . . . . . . . . . . . . . . . 4 2.3 Contributions from rotational motion . . . . . . . . . . . . . . . . . . . . . . 4 2.4 Contributions from vibrational motion . . . . . . . . . . . . . . . . . . . . . 6 3 Thermochemistry output from Gaussian 8 3.1 Output from a frequency calculation . . . . . . . . . . . . . . . . . . . . . . 8 3.2 Output from compound model chemistries . . . . . . . . . . . . . . . . . . . 11 4 Worked-out Examples 11 4.1 Enthalpies and Free Energies of Reaction . . . . . . . . . . . . . . . . . . . . 12 4.2 Rates of Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 4.3 Enthalpies and Free Energies of Formation . . . . . . . . . . . . . . . . . . . 14 5 Summary 17 |
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