Connection between thermodynamic functions and correlation functions
The first approximation is sufficient to obtain the thermodynamic properties of simple liquids and their mixtures, but in order to describe the structural characteristics, subsequent approximations are required that allow us to take into account not only the average density of the system but also the inhomogeneities of the local density. Approximation to the continual description can be achieved both by increasing the number of places occupied by a molecule in the lattice, which is equivalent to a finer division into cells for a molecule of a fixed size, and also taking into account the displacement of the centre of mass of molecules from the centre of the cell. For lattice systems with molecules that block several cells, the general procedure for their statistical analysis based on the cluster approach is given in [65,73]. The breakdown of the volume into cells in the LGM does not impose any restrictions on the motion of the particles; the latter can move around the entire volume. They take into account not only the ‘standard’ positions of the molecules in the centre of the cell, but also the displacements of the particles from the indicated positions, which allows obtaining the structural characteristics of the liquid.