# Selecting a two-component Hamiltonian other than X2C¶

There are several two-component Hamiltonians besides the X2C Hamiltonian (see *.X2C*) implemented
in DIRAC arising from the decoupling transformation of the one-electron DIRAC Hamiltonian.
Likewise spin-orbit interaction terms can be left out and calculations can be performed
in the spin-free mode (i.e. in boson symmetry).

Besides having spin-orbit effects from the decoupling transformation, there is the external AMFI spin-orbit operator which can contribute either with the one-electron spin-orbit operator (accurate to the first order in V, very poor) or with the more valuable mean-field contribution, u_so(1) (also in the first order), which is a reasonable approximation of the ‘Gaunt’ term at the four-component level.

The best two-component Hamiltonian is X2C+MFSSO (similar to BSS+MFSSO or IOTC+MFSSO) where one-electron scalar and spin-orbit effects are up to infinite order, and AMFI MFSSO contributions (mean-field spin-same orbit) provide a ‘screening’ of one-electron spin-orbit terms.

## Examples¶

Both scalar and spin-orbit relativistic effects up to infinite order:

```
.BSS
099
```

Scalar relativistic effects of type “from the beginning” up to the infinite order, no spin-orbit interaction:

```
.SPINFREE
.BSS
109
```

Scalar relativistic effects “from the end” up to the infinite order:

```
.SPINFREE
.BSS
009
```

Traditional scalar relativistic “from the beginning” second-order Douglas-Kroll-Hess Hamiltonian:

```
.SPINFREE
.BSS
102
```

Scalar relativistic “from the end” second-order Douglas-Kroll-Hess Hamiltonian:

```
.SPINFREE
.BSS
002
```

Douglas-Kroll-Hess Hamiltonian with first-order spin-orbit and second-order scalar relativistic effects:

```
.BSS
012
```

Douglas-Kroll-Hess Hamiltonian with second-order spin-orbit and second-order scalar relativistic effects:

```
.BSS
022
```

In the two-component variational scheme is possible to combine external AMFI
spin-orbit terms, [Ilias2001], together with BSS integrals - see the
**AMFI* section. Note that AMFI provides only one-center atomic
integrals.

Infinite order scalar and spin-orbit terms, (one-electron) mean-field spin-same-orbit (MFSSO2) from AMFI:

```
.BSS
2999
```

This is in fact the ‘maximum’ of two-component relativity, resembling Dirac-Coulomb Hamiltonian.

Second-order Douglas-Kroll-Hess spin-free from ‘the beginning’ with first order spin-orbit (SO1) term plus mean-field spin-same-orbit (MFSSO) from AMFI:

```
.BSS
2112
```

SO1 may come either from BSS-transformation or from AMFI. In the latter case it is only for one-center and for point nucleus.

Infinite order scalar and spin-orbit terms, (one-electron) mean-field spin-same and spin-other-orbit (MFSO2) from AMFI:

```
.BSS
3999
```

This mimics the Dirac-Coulomb-Gaunt Hamiltonian, as the spin-other-orbit (SOO) term comes from the Gaunt interaction term.

DIRAC allows to switch off AMFI spin-orbit contributions from various centers.
See the keyword *.NOAMFC*.

Infinite order scalar terms “from the beginning”, (one-electron) spin orbit (SO1) and the mean-field spin-same-orbit (MFSSO2) terms exclusively from AMFI:

```
.BSS
4109
```

Second order (DKH) scalar terms “from the beginning”, (one-electron) spin orbit (SO1) and the mean-field spin-same-orbit (MFSSO2) terms exclusively from AMFI:

```
.BSS
4102
```

Infinite order scalar terms “from the end”, (one-electron) spin orbit (SO1) and the mean-field spin-same-orbit and spin-other-orbit (MFSO2) terms from AMFI:

```
.BSS
5009
```

Infinite order scalar terms “from the end”, and the (one-electron) spin orbit term (SO1) from AMFI:

```
.BSS
6009
```

AMFI one-electron spin-orbit terms - SO1 - are currently for the point nucleus.

For the BSS value ‘axyz’ of y=0, DIRAC employs spin-free picture change transformation of property operators, although the system is not in the boson (spin-free) symmetry for a>1.

Rough first order, DKH1 (not recommended for practical calculations):

```
.BSS
111
```

For comparison purposes between BSS-SO1 and AMFI-SO1 atomic one-center integrals (point nucleus only) use:

```
.BSS
001
```