MOLECULAR-DYNAMICS SIMULATIONS OF 2-M AQUEOUS UREA SOLUTIONS

Results from molecular dynamics simulations of 2 m aqueous urea soluti ons are presented. A recently derived set of ab initio intermolecular potentials with explicit atomic polarizabilities is employed. In the s imulated systems consisting of 10 urea molecules and 277 waters, many different urea-urea complexes are found, in contradiction to other rec ent studies where empirical force fields have been used. These differe nces are due to the inability of the empirical force fields to properl y assign the global urea dimer energy minimum. Four simulations with d ifferent initial starting configurations were carried out in order to ascertain whether the systems are capable of describing the dynamics o f urea dimerization processes. Since we found complex life times and o rientational decay times that are longer than the simulation time (88 ps), it was not possible to achieve equilibrium with respect to the nu mber and character of urea-urea complexes. However, comparison with ne utron scattering experiments gives that the partial radial distributio n functions G(N)(r), G(HH)(r), and G(H(C,O,N)) are insensitive to the degree of urea complexation and therefore also to the intermolecular p otential adopted in a simulation. Future neutron scattering experiment s are proposed, where the presence of urea-urea complexes can be deter mined with certainty.