Electronic structure computations vibrational corrections

The BAC-MP4 method allows the enthalpy of formation of a chemical species to be estimated. The calculation of electronic structures (geometry and vibration frequencies) can be carried out using the GAUSSIAN 94 computer program. The corrections are carried out using the Mdller-Plasset (MP) theory of perturbations and the Bond Additivity Corrections are also carried out using the same theory. 

Overview Over the last few decades, quanmm chemistry has evolved into a predictive tool for the calculation of energy differences. Of equal importance as a driving force for chemical reactions is the entropy that has received less attention from the computational community. The predictive value of computed rate constants depends not only on the accuracy of the electronic structure calculations but also on the correct description of the molecular entropy, including anharmonic effects in the vibrational modes, which is subject of Section 7.3.1.3. 

Secondly, we have not discussed in any detail the effects of nuclear motion. Methods used to calculate these vibrational corrections, for both zero-point vibrational effects and temperature effects, have been described elsewhere in this book. There are, however, other effects that should also be considered. We have not discussed the role of the purely vibrational contributions to molecular (electric) properties (Bishop 1990), which in certain cases can be as large as the electronic contributions (Kirtman et al. 2000). Moreover, in conformationally flexible molecules one has to consider the effects of large nuclear motions. For instance, for a proper comparison with experiment, it may not be sufficient to perform an ab initio calculation for a single molecular structure. In experiment one will always observe the average value of the different thermally accessible isomers (rotamers, conformers), and in order to allow for a direct comparison with these experimental observations, a Boltzmann average of the properties of these isomers must be computed. This is particularly important when the properties of the isomers are very different, possibly even differing in sign (Pecul et al. 2004). 

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