Ab initio Simulations of Ionic Liquids

In ab initio or first-principles simulations, the potential energy of the system and the forces on the atoms result from the solution of the dectronic structure of the system for a fixed nudear configuration. In the study of solid and liquid phases, the behavior of the dectrons is usually described within the framework of density functional theory (DFT) [147,148]. Once the total energy and atomic forces are known, any traditional simulation method can be used to sample representative nudear configurations, allowing for the calculation of thermodynamics, dynamics and stmctural properties. 

The first ah initio simulation of a room temperature molten salt, dimethylimi-dazolium chloride (]MMIM]Q), appeared at the beginning of 2005 [21]. The work aimed at providing information on the liquid structure, in order to compare with results from classical force-field simulations and neutron diffraction experiments. Urdike non-associating fluids, in ionic liquids the distribution of ions around certain chemical bonds may depend strongly on the instantaneous electronic structure. Therefore, site-site distribution functions and three-dimensional densities may change when passing from a classical to a quantum mechanical description of the interactions. 

For [MMIM]C1, the level of agreement between the experimental stmcture and that obtained from classical simulations was good, although some differences were evident in the three-dimensional distribution of around [MMIM]+ [12,128]. The origin of such differences was unclear since, on the one hand, three-dimensional distributions are not directly accessible from the experiments (they have to be obtained through the numerical procedure EPSR [128]) and, on the other hand, force fields are normally parameterized to gas-phase ah initio calculations of the isolated ions. It is well known that the effective dipole moment of a molecule in a liquid may be significantly different to that in the gas phase. 

The performance of the SIE STA calculation was tested by optimizing the stmcture of the solid and comparing both with an equivalent plane wave calculation and with the experimental X-ray stmcture. The intra molecular stmcture was well described 

In the liquid, a first view of the structure was given by the radial distribution functions calculated from the centers of the ions. The simulations revealed a first 

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