10 Exploring Potential Energy Surfaces: Critical Points and Molecular Dynamics

10.7 Ab Initio Molecular Dynamics

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.