For all-electron calculations, the prefered way is to use either the Douglas-Kroll-Hess (DKH) or eXact-2-Component (X2C) Hamiltonians, the former of which is available up to (in principle) arbitrary order in MOLPRO. DKH is activated by setting any of

`SET,DKROLL=1`

`SET,DKHO=`, (),

`SET,DKHP=`, ()

or for X2C by setting

`SET,DKHO=101`

somewhere in the input before the first energy calculation.

Alternatively, these values can be given as options on the `INT` command:

`INT,[DKROLL=1],DKHO=`,`DKHP`=.

or

`INT,DKHO=101`

The DKH option `DKROLL` is available for compatibility with earlier versions of MOLPRO.
If only `DKROLL=1` is given, the default for `DKHO` is 2. Setting `DKROLL=0`
disables DKH and X2C, independently of the setting of `DKHO`. DKH is also disabled by setting
`DKHO=0`, unless `DKROLL=1` is set. In order to avoid confusion, it is recommended
only to use `DKHO` and never set `DKROLL`.

The value of `DKHP` specifies the parametrization for the DKH treatment (it has no effect for X2C):

`DKHP=1`:- Optimum parametrization (OPT, default)
`DKHP=2`:- Exponential parametrization (EXP)
`DKHP=3`:- Square-root parametrization (SQR)
`DKHP=4`:- McWeeny parametrization (MCW)
`DKHP=5`:- Cayley parametrization (CAY)

**Example:**

SET,DKHO=8 |
! | DKH order = 8 |

SET,DKHP=2 |
! | choose exponential parametrization for unitary transformations (recommended) |

Up to fourth order (`DKHO=4`) the DKH Hamiltonian is independent of the chosen
parametrization.
Higher-order DKH Hamiltonians depend slightly on the chosen paramterization of the unitary
transformations applied in order to decouple the Dirac Hamiltonian.

For details on the infinite-order DKH Hamiltonians see

M. Reiher, A. Wolf, JCP **121**, 2037-2047 (2004),

M. Reiher, A. Wolf, JCP **121**, 10945-10956 (2004).

For details on the different parametrizations of the unitary transformations see

A. Wolf, M. Reiher, B. A. Hess, JCP **117**, 9215-9226 (2002).

The current implementation is the polynomial-cost algorithm by Peng and Hirao:
D. Peng, K. Hirao, JCP 130, 044102 (2009).

A detailed comparison of the capabilities of this implementation as well as the current
implementation of the X2C approach is provided in:

D. Peng, M. Reiher, TCA 131, 1081 (2012).

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molpro@molpro.net 2018-01-18