Spin-Orbit coupling

The Spin-Orbit option uses double-group symmetry. The symmetry-adapted orbitals are labeled by the quantum number J rather than L and any references in input to subspecies, such as a specification of occupation numbers, must refer to the double group labels.

Create runs must not use the Spin-Orbit formalism. The SFO analysis of Molecular Orbitals for a Spin-Orbit calculation is only implemented in the case of a scalar relativistic fragment file, which is the whole molecule. Gradient calculations for the Spin Orbit formalism have not yet been implemented. Therefore, computations of harmonic frequencies calculation and geometry optimizations cannot employ this feature.

In a Spin-Orbit run each level can allocate 2 electrons (times the dimension of the irreducible representation) as in a normal restricted calculation. However, contrary to the normal case these two electrons are not directly associated with spin-α and spin-β, but rather with the more general Kramer's symmetry. Using the unrestricted feature in order to assign different numbers of electrons to a and b spin respectively cannot be applied as such. However, one can use the unrestricted option in combination with the collinear or noncollinear approximation. In that case one should use symmetry NOSYM, and each level can allocate 1 electron.