Approximate Quantum Mechanical Calculation of Thermodynamic Properties

3 Approximate Quantum Mechanical Calculation of Thermodynamic Properties 

While the quantum mechanical calculation of the AG i, is reasonably well established, the term with the greatest uncertainty, which therefore limits the accuracy of the method, is the cavity formation term AG, . Various approximations have been made for this term, including the use of spheres and nonspherical shapes to represent the solute with atomic radii empirically adjusted or based on crystal structure. However, as this term is less important for mixture properties (as for a solute in its pure liquid and in the solvent to some extent cancel), PCM has been more useful for the calculation of activity coefficients, octanol-water partition coefficients 

Computational quantum mechanics continues to be a rapidly developing field, and its range of application, and especially the size of the molecules that can be studied, progresses with improvements in computer hardware. At present, ideal gas properties can be computed quite well, even for moderately sized molecules. Complete two-body force fields can also be developed from quantum mechanics, although generally only for small molecules, and this requires the study of pairs of molecules in a large number of separations and orientations. Once developed, such a force field can be used to compute the second virial coefficient, which can be used as a test of its accuracy, and in simulation to compute phase behavior, perhaps with corrections for multibody effects. However, this requires major computational effort and expert advice. At present, a much easier, more approximate method of obtaining condensed phase thermodynamic properties from quantum mechanics is by the use of polarizable continuum models based on COSMO calculations. 

Morokuma, in Applications of Molecular and Materials Modeling. P. R. Westmoreland et al. (eds), Kluwer Academic, New York, 2002. 

Molecular Modelling Principles and Applications, 2nd edn, Prentice Hall, New York, 2001. 

---