Hi,

For relativistic spin-orbit calculations, ADF will accept input
with both "restricted" and "unrestricted" keywords. For "restricted"
calculation, I end up with a set of singly occupied orbitals, which
have (at least in principle) contributions from both spin-alpha
and spin-beta basis functions. Naively, this is that I would expect
spin-orbit coupling operator to do - so all seems to be fine.

However, I am not able to understand what ADF is doing for the "spin-
unrestricted" spin-orbit case (and ADF documentation on spin-orbit
calculations is sketchy at best). The outcome of the calculation is
a set of half-occupied orbitals. At least for the point group I am
working with (Cs), these orbitals are always paired, so that each
half-occupied orbital has a matching orbital, from a conjugated
spinor irrep. The energies of these pairs closely match energies of
the orbitals from scalar relativistic calculation.

The only intepretation I was able to come up with, is that in a
"spin-unrestricted" calculations, orbitals from conjugated irreps
are constrained to form a linear combination with a value of sz
of +/-0.5; these linear combination then form a non-interacting
determinant with the value of Sz prescribed by the "CHARGE" keyword.
If this is true, a "spin-unrestricted" calculation does NOT lead
(except by accident) to proper eigenfuctions of spin-orbit Hamiltonian,
but rather to some mixed state.

So, now come the questions:

1. Is this interpretation of a "spin-unrestricted spin-orbit"
   calculation in ADF close to the truth?

2. If it is, what is the physical meaning of such calculation?
   What information can be extracted from it?

3. Can somebody point me to a publication, documenting this
   technique?

Thank you!

Serguei

---
Dr. Serguei Patchkovskii
Tel: +1-(613)-991-2719
Fax: +1-(613)-947-2838
E-mail: Serguei.Patchkovskii_at_nrc.ca
Research Council Officer
Theory and Computation Group
Steacie Institute for Molecular Sciences
National Research Council Canada
Room 2158, 100 Sussex Drive
Ottawa, Ontario
K1A 0R6 Canada