Speeding up SCC MD

[Input: recipes/moleculardynamics/XLBOMD/]

Instead of standard Born-Oppenheimer (BO) molecular dynamics, there are several alternative methods that can propagate both the atomic positions and electronic structure simultaneously, often with substantial improvements in computational speed.

These are either the Car-Parrinello scheme or extended Lagrangian (XL) Born-Oppenheimer dynamics. DFTB+ supports the second of these methods, which (when stable) produces SCC equivalent dynamics to conventional Born-Oppenheimer molecular dynamics but with similar (lower) costs to non-SCC DFTB.

This example uses the fast XL-BOMD method, with a single SCC step for each geometry once the calculation starts. Here the dynamics of an extended Lagrangian is used to predict the charges for each time step, but without using a self-consistent loop. Fast XL-BOMD is potentially less stable, so should only be used for systems which show good SCC behaviour (molecules without degeneracies or solids with wide band gaps).

To enable XL dynamics, the VelocityVerlet{} block should be modified to contain an extra section:

XlbomdFast {
    IntegrationSteps = 5
    Scale = 0.5
    TransientSteps = 10
}

This instructs the code to use 5 time steps to integrate the equations of motion (EOM) for the system with an initial 10 steps of Born-Oppenheimer dynamics before starting the XL dynamics. The initial BO steps lead to a more stable start for the XL dynamics. The scaling factor is used to also increase stability of the XL-EOMs, and should be in the range of (0..1], where the largest value which is stable should be used (this can only be determined by testing).

Due to the reduced requirements on SCC during XL-BOMD dynamics, the standard DFTB forces are usually not sufficiently accurate due to increased self-consistency errors. Hence an extra command should be added to the Hamiltonian {} block:

ForceEvaluation = Dynamics

This enables an additional correction in the forces, which can be used for Fermi fillings. This correction can also be used in other calculations where the forces are evaluated with limited self-consistency (for example to speed up geometry relaxation be relaxing the SCCTolerance).

In the special case of the electronic temperature of the system being set to be 0, there is a faster version of this correction which can be used:

ForceEvaluation = DynamicsT0