Thermochemistry, computational

Computer-assisted thermochemistry is a tool that can be applied in many fields today. In particular, with the aid of reliable thermochemical source data and appropriate application software, optimum operating tempera tures, reacting amounts and/or gas pressures necessary to obtain a product of the required purity can be calculated. Costly and time-consuming experimental work can thereby be reduced considerably [1]. 

The flame temperature of Example 3.3 will be calculated with the program ChemSage. In the first calculation the assumption is that the following reaction takes place  

It is also possible with the ChemSage program to calculate the adiabatic flame temperature without neglecting possible by-products . In this case the 

A The header contains values for temperature, pressure and volume of the system. In this particular case, the temperature is a calculated result based on the constraint of adiabatic conditions. 

B The reactants are given by name and phase (where appropriate), and their overall amounts (column 2). If, as here, there is an extensive property calculation the initial temperature (column 3) and pressure (column 4) are also defined. 

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Irikura, K. K. Essential Statistical Thermodynamics in Computational Thermochemistry, In Irikura, K. K. Erurip, D. J. Eds., 1998. Computational Thermochemistry. American Chemical Society, Washington, DC. 

In principle, it should be possible to use computational thermochemistry to calculate free energies of formation for unknown tetrahedral intermediates. In practice this remains difficult because of the problem of estimating solvation energies. There is no doubt that computational methods will become increasingly important in this as in other areas. 

K. K. Irikura, D. J. Frurip, Eds. Computational Thermochemistry Prediction and Estimation of Molecular Thermodynamics. ACS Symposium Series 677 American Chemical Society Washington, DC, 1998. 

There would thus appear to be room for a more or less black box computational thermochemistry method that has the following properties ... 

In this chapter, we review the elements of G3 theory and related techniques of computational thermochemistry. This review is restricted almost exclusively to the techniques that we have developed and the reader is referred to the remaining chapters in this volume for other complementary approaches. An important part of the development of such quantum-chemical methods is their critical assessment on test sets of accurate experimental data. Section 3.2 provides a brief description of the comprehensive G3/99 test set [26] of experimental data that we have collected. Section 3.3 discusses the components of G3 theory as well as the approximate versions such as G3(MP3) [22] and G3(MP2) [23], and their performance for the G3/99 test set. The G3S method [29] that includes multiplicative scale factors is presented in section 3.4 along with other related variants. Section 3.5 discusses the recently developed G3X method [30] that corrects for most of the deficiencies of G3 theory for larger molecules. The performance of these methods is compared to... 

A new family of methods, referred to as G3S (G3 Scaled), has been developed recently [29], where the additive higher-level correction is replaced by a multiplicative scaling of the correlation and Hartree-Fock components of the G3 energy. The scale factors have been obtained by fitting to the G2/97 test set of energies. This test set is substantially larger than that used in previous fits and can provide a reliable assessment of the use of such a scaling approach to computational thermochemistry. 

U. Burkert and N. L. Allinger, Molecular Mechanics, American Chemical Society, Washington, DC 1982 D. W. Rogers, Computational Chemistry Using the PC, 2nd ed., VCH Publishers, New York, 1994, Chapter 10 D. W. Rogers, in Computational Thermochemistry, K. K. Irikura and D. J. Frurip, eds., American Chemical Society, Washington, DC, 1998, Chapter 7 D. M. Hirst, A Computational Approach to Chemistry, Blackwell Scientific Publications, Oxford, UK, 1990, Chapter 3 ... 

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 ... 

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