MOTIVATION AND GENERAL SETTING

Molecular modeling includes a collection of computer-based tools of varying theoretical soundness, which make it possible to explain, and eventually predict, the properties of molecular systems on the basis of their composition, geometry, and electronic structure. The need for such modeling arises while studying and/or developing various chemical products and/or processes. The raison d etre of molecular modeling is provided by chemical thermodynamics and chemical kinetics, the basic facts of which are assumed to be known to the reader.1 

According to chemical thermodynamics the relative stability of chemical species and thus their basic capacity to transform to each other (understood in a very wide sense, for example, as the possibility to form solutions i.e. homogeneous mixtures with each other or undergo phase transitions e.g. from gas to liquid state) is described by the equilibrium constant of the (at this point) hypothetical process  

1The sources in physical chemistry are numerous. Elementary volumes to be known by heart are [1,2] the more the better. 

If the set of species Pi, P2. on the left of eq. (1.1) is more stable than Pi, P2. on the right, the equilibrium constant shows that by being less than one Keq 1. In this case the system prepared as a mixture of species Pi, P2. with [Pi] = [P2] =. .. = 0 tends to stay in the state where the concentrations of reactants are generally larger than those of the products, although of course in the equilibrium described by eq. (1.2) all species are present in concentrations necessary to satisfy this equation. If the opposite happens and Keq 1 the system prepared as a mixture of the species Pi, P2. tends to transform to that formed predominantly by the species Pi, P2. to the extent prescribed by the previous inequality and eq. (1.2). 

The magnitude of Keq is governed by the relative value A AG/ for the left and right sides of eq. (1.1) of a single quantity - the Gibbs free energies of formation of the system under study in its left hand and right hand states  

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