Computer experiments on aqueous solutions. VII. Potential energy function for urea dimer and molecular dynamics calculation of 8 mol % aqueous solution of urea

Molecular dynamics calculation (MD) has been carried out for an aqueous solution of urea at 298.15 K and with experimental density value at ordinary pressure by the use of constant temperature technique developed previously. The total number of molecules is 216, of which 17 are urea. The mole fraction of urea in the solution is thus 0.078. For water-water and water-urea interactions, the MCY potential and previously determined potential have been used. A new urea-urea pair potential is determined by ab initio LCAO SCF calculations for more than 750 different dimer configurations with an STO-3G basis set and subsequent multiparameter optimization of the MO data to a 12-6-3-1 potential energy function. The MD calculation is extended up to 76 000 time steps and final 44 000 time steps (17.6 ps) are used to calculate both static and dynamic properties. Among other information, the following results are important and interesting: (1) Urea molecules exhibit appreciable self-association, (2) such association is possible with strong hydrogen bondings and this is in sharp contrast with the association of t-butanol through hydrophobic interaction studied previously, (3) there is no evidence that urea destroys structure of water, and (4) the decrease of self-diffusion coefficient of water in urea solution is appreciably smaller than that in the case of t-butanol solution.