The number of plane waves included in the calculation is controlled by the cutoff energy .

`cut_off_energy : value [units]`

Default `[units]` are eV.

Increasing the value of this parameter increases the computational cost of the calculation. However,
is a variational parameter and the total energy will convergence towards the variational minimum
as
is increased. It is crucial to repeat the calculation a few times with a fixed k-point grid but with
increasing
. 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
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.