Molecular system dispersion correction

In the theory of electric properties of molecular systems in degenerate electronic states some unsolved problems remain. First, the problem of intermolecular interactions considering the degeneracy of the electronic states of the interacting molecules has not been solved completely. In this case, besides the lowering of the multipolarity of the interaction described in this paper, one can expect an essential contribution of anisotropic induction and dispersion interactions to different virial correction to the equations of state, refraction, and other electric characteristics of matter. 

It is noteworthy that as part of the same study, a crystal structure prediction of phenylethylammonium lactate was also carried out. The results were less impressive than the results obtained for pyridinium chloride the experimentally observed structure of phenylethylammonium lactate was found as the fifth crystal packing alternative, 0.24 kcal mol above the global lattice energy minimum structure. The reasons for this discrepancy are unclear. It is possible that the experimental structure of phenylethylammonium lactate is a kinetic polymorph and a thermodynamically more stable structure has not been found yet, or the known structure is stabilised by temperature effects that were not considered in the simulations, or the parameters of the dispersion correction do not transfer well from neutral systems to charged systems. Further validation work on molecular salts is required to pinpoint the reason for the inconclusive results obtained for these two compounds. 

In summary, various approaches were developed and validated for ionic liquids over the last years. It was shown that dispersion corrected KS-DFT approaches allow reliable results for ionic liquids. " Recently, a comparison of trajectories obtained from ab initio molecular dynamics simulations with and without a dispersion correction revealed that the dynamics of the system is more accurately described for the dispersion corrected one, ° which highlights the necessity of dispersion corrected approaches in ab initio molecular dynamics simulations. Polarisable force fields were developed which allow the investigation of various ionic liquids.Cheaper nonpolarisable force fields are also available for a broad range of ionic liquids. However, the latter force fields tend to show too... 

The ab initio methods used by most investigators include Hartree-Fock (FFF) and Density Functional Theory (DFT) [6, 7]. An ab initio method typically uses one of many basis sets for the solution of a particular problem. These basis sets are discussed in considerable detail in references [1] and [8]. DFT is based on the proof that the ground state electronic energy is determined completely by the electron density [9]. Thus, there is a direct relationship between electron density and the energy of a system. DFT calculations are extremely popular, as they provide reliable molecular structures and are considerably faster than FFF methods where correlation corrections (MP2) are included. Although intermolecular interactions in ion-pairs are dominated by dispersion interactions, DFT (B3LYP) theory lacks this term [10-14]. FFowever, DFT theory is quite successful in representing molecular structure, which is usually a primary concern. 

---