Lennard-Jones parameters refinement

Koddermann et al. calculated the heats of vaporization for imidazolium-based ILs [Cnmim][NTf2] with n = 1, 2, 4, 6, 8 by means of MD simulations [81], The authors applied a force field which they had developed recently. Within this force field the authors reduced the Lennard-Jones parameters in order to reproduce experimental diffusion coefficients [81], The refined force field also led to absolute values of heats of vaporization as well as their increase with the chain length of the imidazolium cation such that this quantities were described correctly. The overall heats of vaporization were split in several contributions and discussed in detail. The authors observed that with increasing alkyl chain length, the Coulomb contribution to the heat of vaporization remained constant at around 80 kJ mol 1, whereas the van der Waals interaction increased continuously. The calculated grow of about 4.7 kJ mol"1 per CH2-group of the van der Waals contribution in the IL exactly matched the increase in the heats of vaporization for n-alcohols and n-alkanes, respectively. The results support the importance of van der Waals interactions even in systems completely composed of ions [81],... 

Based on DFT calculations on chlorophylls and, additionally, on ubiquinone and the RC main detergent, lauryl dimethylamine oxide or LDAO, we have then developed a force field for their classical modelization. Our approach to this undertaking was straightforward. We initially use the DFT optimized structures and the vibrational analysis to determine the bonded part of the potential parameters described by the AMBER potential function. Then, atomic ab initio partial charges on the chromophore are used to account for electrostatic effects. At a later stage, experimental data from X-ray crystallography are used to check the structural properties of the molecule in the condensed state and to refine the intermolecular Lennard-Jones parameters. 

An important feature of the BLCT Eq. 37 is the proper reproduction of the deuterium swap effect, i.e., a larger x is always obtained when the more branched polyolefin component is deuterated. This deuterium swap effect has been found in numerous experiments [28,97,103,104] and follows as a direct consequence of Eq. 37 provided that the self-interaction energy Saa is higher for the less branched polyolefin species. It should be possible to rationalize the latter constraint on the interaction energies from the models employed for off-lattice simulations of alkane systems. These continually refined models use considerably larger Lennard-Jones interaction parameters for CH3 groups than for CH (n = 0,1,2) units. 

The molecular dynamics results found from parameter set 1 are almost the same as those found for the Williams potential using his set VII, and reported elsewhere (10). The results for the rescaled Lennard-Jones potential used here are substantially better, as seen from Table II. We plan to study further refinements of this potential model, including the addition of an octupole moment, and we are also in the process of investigating other properties, including the orientational correlation functions, self-diffusion coefficients, and time correlation functions. 

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