Quantum Chemistry in Engineering Thermodynamics

Abstract

Quantum mechanics (QM) or quantum chemistry (QC) calculations have been extensively used in the past for the calculation of heats of formation and reaction, heat capacities, reaction pathways and transition states. 1 They are currently becoming increasingly popular for estimating phase equilibria and other properties. The starting point in QM, the Schr€ odinger equation, cannot be solved exactly for multielectron systems, and thus approximations are necessary. Deciding upon the method and level that the computations should be made is not a trivial task. Therefore, there are today a number of software packages available for QM calculations, e.g. Gaussian (www.gaussian.com), Schr€ odinger (www.schrodinger.com, distributing the software Jaguar), Turbomole (www.turbomole.com) and Gamess 2 . The packages include various calculation procedures, such as ab initio and density functional methods.The purpose of this chapter is not to review these computational methods, but rather to provide a short presentation and evaluation of certain QM methods which have already found use in engineering applications. For further information on the computational methods, the reader is referred to the references of this chapter, especially the reviews by Sandler and co-workers. 1,3,4 Before presenting the methods that we will highlight in this chapter, it is important to emphasize that QM calculations can be used in different semi-direct or indirect approaches in engineering thermodynamics:1. Calculation of the intermolecular potential from QM and then phase equilibrium calculations using molecular simulation. 2. QM calculations to determine parameters in existing thermodynamic models. 3. The continuum solvation (polarizable) models such as COSMO-RS.The first method will not be discussed in detail in this chapter. It is currently computationally very intensive and limited to rather simple molecules. It has been used to calculate the second virial coefficients and VLE of a few molecules and systems such as methyl fluoride, methanol, acetonitrile and methanol-hydrogen fluoride. The agreement is best for the simpler molecules but for hydrogen bonding compounds such as methanol and hydrogen fluoride, corrections are required. This may be due to the fact that for hydrogen bonding molecules,