Dynam is a program that enables a computer running the Windows operating system to simulate the movement of a group of molecules in three-dimensional space. Version 2 of Dynam takes advantage of the improvements in computer hardware and software that have taken place since the original version was published (1). Dynam now features improved graphics, the capability to simulate larger samples of molecules and the ability to create "virtual reality" files that show the molecular motion in three dimensions.

Dynam assumes that the molecules are nonpolar and subject to Lennard-Jones forces. This means that the attractive force between any two molecules is inversely proportional to the sixth power of the distance between them while the much shorter-range repulsive force is inversely proportional to the twelfth power of that distance.

Limitations of computer memory and speed make it necessary to limit the calculation to a small number of molecules, currently 32, 108, or 256. In order to prevent surface effects from becoming dominant, the program imposes a periodic boundary condition in which the cubic cell containing the molecules repeats itself over and over in space. When a molecule leaves one side of the cell it appears in the opposite side. At any given point in the calculation the nearest neighbors of a given molecule may include molecules in adjoining cells.

Each molecule is assigned an arbitrary initial position together with a random initial speed and direction of movement. The calculation proceeds in a series of extremely small time intervals of the order of femtoseconds. Newton's laws of motion are used to calculate the position and velocity of each molecule at the end of a given time interval from its position and velocity at the beginning. Corrections are made to the positions and velocities of any two molecules as they approach one another. At moderate intermolecular distances they are attracted to each other while at smaller distances they are repelled.

Molecules in the gas phase move in straight lines until they collide with one another. In the liquid phase, molecules move around each other in complex paths. In the solid phase molecules can only oscillate around their equilibrium positions in the crystal lattice.

Dynam shows molecular motion either as moving spheres or as the paths the molecules have taken as they move around. Optional displays include the radial distribution function, which shows the probability of finding neighboring molecules as a function of the distance from a given molecule, and a histogram of molecular velocities. Virtual reality (VRML) files created by the program can be viewed with an Internet browser equipped with a VRML plug-in.