Impact of Advances in Computing and Communications Technologies on Chemical Science and Technology Report of a Workshop (1999) / Chapter Skim
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3 Recent Advances in Computational Thermochemistry and Challenges for the Future
3 Recent Advances in Computational Thermochemistry and Challenges for the Future
Pages 26-34
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From page 26... ...
At the ah initio molecular orbital level, the methods currently used for computing thermochemical data range from very high levels of theory to those that combine moderate levels of theory with some form of empirical input. The former is limited to smaller molecules and can attain accuracies of +0.5 kc al/ mol.
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From page 27... ...
composite techniques that combine moderate level ah initio quantum chemical calculations with some form of molecule-independent empirical parameters; and (3) techniques that use molecule-dependent empirical parameters obtained from accurate experimental data in combination with moderate level ah initio quantum chemical calculations.
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From page 28... ...
Use of specially chosen reactions can give quite accurate enthalpies of formation because of cancellation of correlation effects and also because they make use of accurate experimental data. However, suitable reference molecules are often not available, making this method inapplicable in many cases.
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From page 29... ...
On the G2 test set of small molecules, G2 theory is able to reproduce adequately (average absolute deviation of 1.2 kcal/mol) the experimental data; however, on certain larger molecules it fails.
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From page 30... ...
The higher-level correction in step 4 is reformulated in terms of four parameters, but remains molecule-independent and is obtained by fitting to a larger set of accurate experimental data. Two new steps are added: a correction for core correlation at the secondorder perturbation level using a new basis set, G31arge, and a correction for spin-orbit effects in atoms.
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From page 31... ...
and accuracy for G3 energy calculations on test molecules containing up to ten carbons.
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From page 32... ...
Most of the computational thermochemistry methods that have been developed so far have focused on molecules containing first- and second-row elements. Thermochemical data on compounds containing elements beyond the second row are important but are more difficult to predict accurately because of the increased importance of relativistic and correlation effects; lack of adequate basis sets; and lack of accurate experimental data for assessments.
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From page 33... ...
A difficulty there is that the good experimental data is quite limited. Part of the trouble is that doing thermochemistry experimentally went out of fashion some time ago and there is less new material appearing than used to be there.
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From page 34... ...
The biggest relativistic correction that we have found is for silicon tetrafluoride, which is about 3 kcal/mole. So, if you include relativity you would have to reparameterize these small parameters.
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