Computer simulations of liquid water: treatment of long-range interactions

Computer simulations by the molecular-dynamics method are used to study the physical properties of liquid water. Two three-point-charge models (SPC and TIPS) are analysed and compared using the Ewald-Kornfeld summation method to calculate long-range electrostatic interactions. Although these two models are not very different considering their geometry and energy parameters, they lead to different physical properties of liquid water. Simulations with the SPC water model and the Ewald-summation method are then used as a reference against which different termination functions for Coulombic interactions are compared. The largest discrepancies between simulations performed using the truncated potentials combined with termination functions and those performed using the non-truncated one occur in the radial distribution functions and in the diffusion coefficients. The origin of these discrepancies is analysed and it is shown that the termination function of the form 1 - 2(r/r c) n + (r/r c)2n with n = 1 provides results most resembling those obtained using the Ewald summation.