Molecular dynamics simulations are a computational model of a collection of molecules that interact with one another under a specified set of conditions (temperature, pressure, etc.). While electronic structure theory calculations typically only involve one or a few molecules, molecular dynamics simulations can involve hundreds or thousands of molecules all at once. The purpose of molecular dynamics simulations is typically to investigate how molecules interact and react while surrounded by a collection of other molecules. While the energies of individual molecules can be investigated by electronic structure theory, a molecular dynamics simulation allows theoretical chemists to also study how molecules are influenced by their surroundings. These effects can be especially important in liquids, where solvent molecules can have a significant impact on reactivity.