Basis Set Parameters

The number of plane waves included in the calculation is controlled by the cutoff energy $E_{cutoff}$.

cut_off_energy : value [units]

Default [units] are eV.

Increasing the value of this parameter increases the computational cost of the calculation. However, $E_{cutoff}$ is a variational parameter and the total energy will convergence towards the variational minimum as $E_{cutoff}$ is increased. It is crucial to repeat the calculation a few times with a fixed k-point grid but with increasing $E_{cutoff}$. The total final energy will asymptotically reduce to the variational minimum and when it levels off the user can be confident that they have the right value for $E_{cutoff}$ and that the calculation will be properly converged.

How do we decide what cutoff energy to use when first approaching a new system? Trial and error is not feasible for large jobs. There is a parameter which will automatically choose a cutoff energy

basis_precision : value

value can be coarse, medium, fine, precise or extreme. For fixed cell calculations it is suggested that the basis precision should be at least fine and for variable cell calculations it should be at least precise but the user should carefully check in both cases.