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(June 30, 2021)

Q-Chem can propagate classical molecular dynamics trajectories on the
Born-Oppenheimer potential energy surface generated by a particular theoretical
model chemistry (*e.g.*, B3LYP/6-31G* or MP2/aug-cc-pVTZ). This
procedure, in which the forces on the nuclei are evaluated on-the-fly, is known
variously as “direct dynamics”, “*ab initio* molecular dynamics”
(AIMD), or “Born-Oppenheimer molecular dynamics” (BOMD). In its most
straightforward form, a BOMD calculation consists of an energy + gradient
calculation at each molecular dynamics time step, and thus each time step is
comparable in cost to one geometry optimization step. A BOMD calculation may be
requested using any SCF energy + gradient method available in Q-Chem,
including excited-state dynamics in cases where excited-state analytic
gradients are available. As usual, Q-Chem will automatically evaluate
derivatives by finite-difference if the analytic versions are not available for
the requested method, but in AIMD applications this is very likely to be
prohibitively expensive.

While the number of time steps required in most AIMD trajectories dictates that economical (typically SCF-based) underlying electronic structure methods are required, any method with available analytic gradients can reasonably be used for BOMD, including (within Q-Chem) HF, DFT, MP2, RI-MP2, CCSD, and CCSD(T). The RI-MP2 method, especially when combined with Fock matrix and $Z$-vector extrapolation (as described below) is particularly effective as an alternative to DFT-based dynamics.