# Relativistic Effects and Spin¶

## Spin polarization¶

By default Band calculations are spin-restricted. You can instruct Band to perform a spin-unrestricted via the `Unrestricted`

key:

```
Unrestricted Yes/No
```

`Unrestricted`

- Type
Bool

- Default value
No

- Description
Controls whether Band should perform a spin-unrestricted calculation. Spin-unrestricted calculations are computationally roughly twice as expensive as spin-restricted.

The orbitals are occupied according to the aufbau principle.

If you want to enforce a specific spin-polarization (instead of occupying according to the aufbau principle) you can use the `EnforcedSpinPolarization`

key:

```
EnforcedSpinPolarization float
```

`EnforcedSpinPolarization`

- Type
Float

- GUI name
Spin polarization

- Description
Enforce a specific spin-polarization instead of occupying according to the aufbau principle. The spin-polarization is the difference between the number of alpha and beta electron. Thus, a value of 1 means that there is one more alpha electron than beta electrons. The number may be anything, including zero, which may be of interest when searching for a spin-flipped pair, that may otherwise end up in the (more stable) parallel solution.

## Relativistic Effects¶

Relativistic effects are treated with the accurate and efficient ZORA approach 1 2, controlled by the `Relativity`

keyword. Relativistic effects are negligible for light atoms, but grow to dramatic changes for heavy elements. A rule of thumb is: Relativistic effects are quite small for elements of row 4, but very large for row 6 elements (and later).

```
Relativity
Level [None | Scalar | Spin-Orbit]
End
```

`Relativity`

- Type
Block

- Description
Options for relativistic effects.

`Level`

- Type
Multiple Choice

- Default value
Scalar

- Options
[None, Scalar, Spin-Orbit]

- GUI name
Relativity (ZORA)

- Description
None: No relativistic effects. Scalar: Scalar relativistic ZORA. This option comes at very little cost. SpinOrbit: Spin-orbit coupled ZORA. This is the best level of theory, but it is (4-8 times) more expensive than a normal calculation. Spin-orbit effects are generally quite small, unless there are very heavy atoms in your system, especially with p valence electrons (like Pb). See also the SpinOrbitMagnetization key.

See also the SpinOrbitMagnetization key.

References

- 1
P.H.T. Philipsen, E. van Lenthe, J.G. Snijders and E.J. Baerends,

*Relativistic calculations on the adsorption of CO on the (111) surfaces of Ni, Pd, and Pt within the zeroth-order regular approximation.*Physical Review B 56, 13556 (1997).- 2
P.H.T. Philipsen, and E.J. Baerends,

*Relativistic calculations to assess the ability of the generalized gradient approximation to reproduce trends in cohesive properties of solids.*Physical Review B 61, 1773 (2000).