A method in computational chemistry to investigate chemical systems in a time dependent way. Usually this involves evaluating the forces in the atoms, letting the atoms take a step in response to those forces, and repeating, over and over again.

Starting with the coordinates of the atoms, a first push is sometimes needed to get the ball rolling. This perturbation is then carried through via the gradient calculations. That is, at any time (t) the forces (x,y,z) for each atom are calculated and the motion is simulated by adding or subtracting to the coordinates. Hard sphere models pretend atoms are billiard balls that bounce off each other, while more realistic descriptions of the forces (Van der Waals for example) take longer to calculate. To really slow down the simulation, electrostatics are taken into account - these only drop off with 1/r, so potentially each atom is affected by every other atom!