molecular dynamics

Molecular dynamics, or MD, is like running a tiny stop-motion movie of every atom in a protein and its surrounding water, frame by frame. Starting from a structure, the computer calculates the forces on each atom and nudges them forward a tiny step in time, then repeats this millions of times to watch how the system wiggles, breathes, and rearranges.

The engine of MD is a force field, a set of physics-based equations and parameters that approximate the forces between atoms from bonds, angles, charges, and van der Waals contacts. Each step advances time by roughly one or two femtoseconds, so reaching even a microsecond of simulated motion requires hundreds of millions of steps. MD reveals things a static crystal structure cannot: how a pocket opens and closes, where ordered waters sit, and whether a ligand stays bound or drifts.

The honest limits are timescale and accuracy. Many biologically interesting motions happen on millisecond timescales that remain expensive to simulate, and results depend on the chosen force field and starting structure. MD is therefore powerful for mechanism and hypothesis, and it provides the conformational sampling that underpins free-energy calculations, but its numbers should be read with their assumptions in mind.