For consideration of scalar-relativistic effects in all-electron calculations the scalar-relativistic Douglas-Kroll-Hess (DKH)
Hamiltonian can be employed in the modules `dscf` and `ridft`.
Please make sure to use an all-electron basis set (in particular, ECPs are not allowed).

This Hamiltonian is defined up to a certain order in the external potential and is available up to arbitrary order (for actual calculations, however, it is advisable not to go beyond 4th order (the parameter settings of the implementation allow to run calculations up to about 10th order in the electron-nucleus potential).

The current implemtation is restricted to single-point calculations (gradients are not available) in C1-symmetry and cannot be used in parallel mode. Moreover, calculated properties using the DKH density do not take care of the 'picture-change' effect.

The arbitrary-order Hamiltonian requires in the control file:

where *integer* specifies the order of the DKH Hamiltonian in the external potential,
i.e. for the standard 2nd-order DKH Hamiltonian one should use `$dkhorder 2`.

The parametrization of the unitary transformation used in the DKH transformation can be optionally selected by

The possible parametrizations in the DKH transformation are:

`$dkhparam 1`:- Optimum parametrization (OPT)
`$dkhparam 2`:- Exponential parametrization (EXP)
`$dkhparam 3`:- Square-root parametrization (SQR)
`$dkhparam 4`:- McWeeny parametrization (MCW)
`$dkhparam 5`:- Cayley parametrization (CAY)

For details on the arbitrary-order DKH Hamiltonians see Ref. [64] for details on the infinite-order DKH theory, [65] for the implementation, and [66] for a conceptual review of DKH theory. For details on the different parametrizations of the unitary transformations see [67].