# 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 [True | False]

Unrestricted
Type: Bool False Controls wheather 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 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 [17, 18], controlled by the Relativistic 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 Options for relativistic effects.
Level
Type: Multiple Choice None [None, Scalar, Spin-Orbit] 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.