Our algorithm, with the judicious use of fairly short range cut-offs should provide a natural next step in that direction, with a substantially better model of the short range force responsible for hydrogen bonding. The TIP4P model includes a Lennard Jones 6-12 interaction between the oxygen mass centers of the water molecules and electrostatic interactions between the charged sites. The molecules are confined in a cubic simulation box whose size depends on the density of the simulated material. The molecules are initially assigned random locations within the box and given an initial random linear and angular velocities and orientation. At every time step, the force on each atom due to all other molecules is calculated.
Newtonian mechanics is used to move each atom in the direction of the resultant force.
We implement a velocity-Verlet integration scheme for the translation motion. The simulation volume is divided into fictitious cells whose length is greater than or equal to the force cutoff distance. Thus, a molecule in a given cell interacts only with the other molecules in its cell and in the 26 neighboring fictitious cells. The cell-list is constructed at the beginning of the calculation, and each cell contains a pointer list to its 26 neighboring cells. In Fortran 95, in which our code was written, the cell-list is easily updated before each force calculation through the use of pointers.