4.7.3 Dual-Basis Dynamics

The ability to compute SCF and MP2 energies and forces at reduced cost makes dual-basis calculations attractive for ab initio molecular dynamics simulations, which are described in Section 9.9. Dual-basis BOMD has demonstrated¹⁰⁷³ savings of 58%, even relative to state-of-the-art, Fock-extrapolated BOMD. Savings are further increased to 71% for dual-basis RI-MP2 dynamics. Notably, these timings outperform estimates of extended Lagrangian (“Car-Parrinello”) dynamics, without detrimental energy conservation artifacts that are sometimes observed in the latter.⁴⁵⁴

Two algorithm improvements make modest but worthwhile improvements to dual-basis dynamics. First, the iterative, small-basis calculation can benefit from Fock matrix extrapolation.⁴⁵⁴ Second, extrapolation of the response equations (“$Z$-vector” equations) for nuclear forces further increases efficiency.¹⁰⁷⁶ (See Section 9.9.) Q-Chem automatically adjusts to extrapolate in the proper basis set when DUAL_BASIS_ENERGY is activated.