Editing Molecular dynamics (section)

=== Potentials in ''ab initio'' methods ===
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In classical molecular dynamics, one potential energy surface (usually the ground state) is represented in the force field. This is a consequence of the [[Born–Oppenheimer approximation]]. In excited states, chemical reactions or when a more accurate representation is needed, electronic behavior can be obtained from first principles using a quantum mechanical method, such as [[density functional theory]]. This is named ''Ab Initio Molecular Dynamics'' (AIMD). Due to the cost of treating the electronic degrees of freedom, the computational burden of these simulations is far higher than classical molecular dynamics. For this reason, AIMD is typically limited to smaller systems and shorter times.

''[[Ab initio]]'' [[quantum mechanical]] and [[Quantum chemistry|chemical]] methods may be used to calculate the [[potential energy surface|potential energy]] of a system on the fly, as needed for conformations in a trajectory. This calculation is usually made in the close neighborhood of the [[reaction coordinate]]. Although various approximations may be used, these are based on theoretical considerations, not on empirical fitting. ''Ab initio'' calculations produce a vast amount of information that is not available from empirical methods, such as density of electronic states or other electronic properties. A significant advantage of using ''ab initio'' methods is the ability to study reactions that involve breaking or formation of covalent bonds, which correspond to multiple electronic states. Moreover, ''ab initio'' methods also allow recovering effects beyond the Born–Oppenheimer approximation using approaches like [[mixed quantum-classical dynamics]].