Accelerated MD with response equation extrapolation
Ab initio molecular dynamics (MD) involves the classical propagation of trajectories on the Born-Oppenhimer potential energy surface. Though traditionally performed with DFT-based methods, DFT has known failures (most notably charge transfer and dispersion complexes) that many times require a wavefunction-based treatment of the electronic structure. Such methods are computationally expensive, however, and methods to make them tractable for MD are needed.

Extrapolation of the SCF Hamiltonian (Fock matrix) has been shown to significantly reduce the cost of SCF (HF and DFT) calculations without biasing the resulting dynamics. In this work, we showed that similar techniques can be applied to the so-called "z-vector" response equation in correlated-wavefunction MD. Results were demonstrated for MP2-based dynamics, but the method is general and can be equally applied to coupled-cluster or even dual-basis MD.

An important detail to the implementation was the use of the AO-based z-vector density, instead of the MO-based z-vector itself. The latter can undergo orbital-ordering oscillations, even when the electronic state is smoothly changing. The resulting, extrapolated AO-based density, however, is in excellent agreement with the converged z-vector density, and only 1–2 z-vector cycles were required.