Handbook of Solvents - George Wypych - ChemTech - Ventech!

40 Koichiro Nakanishi
Figure 2.2.1. A scheme for the design of molecular ensembles.
sion of additivity rule, which has now an empirical character. It is however useful for practical
purposes.
2.2.5 MODEL SYSTEM AND PARADIGM FOR DESIGN
The method described above is so to speak an orthodox approach and the ability of present-day’s
supercomputer is still a high wall in the application of molecular simulation. Then
the role of the second method given in the last section is highly expected.
It is the method with empirical potential model. As the model, the approximation that
any molecule can behave as if obeying Lennard-Jones potential seems to be satisfactory.
This (one-center) LJ model is valid only for rare gases and simple spherical molecules. But
this model may also be valid for other simple molecules as a zeroth approximation. We may
also use two-center LJ model where interatomic interactions are concentrated to the major
two atoms in the molecule. We expect that these one-center and two-center LJ models will
play a role of Occam’s razor.
We propose a paradigm for physical properties prediction as shown in Figure 2.2.1.
This corresponds to the stage (2) and may be used to prepare the process of stage (3),
namely, the molecular ensemble design for solvents.
Main procedures in this paradigm are as follows; we first adopt target molecule or
mixture and determine their LJ parameters. At present stage, LJ parameters are available
only for limited cases. Thus we must have method to predict effective LJ parameters.