  An efficient perturbational treatment of spin-orbit coupling within the
framework of high-level multi-reference techniques has been implemented in the
most recent version of the COLUMBUS quantum chemistry package, extending the
existing fully variational two-component (2c) multi-reference configuration
interaction singles and doubles (MRCISD) method. The proposed scheme follows
related implementations of quasi-degenerate perturbation theory (QDPT) model
space techniques. Our model space is built either from uncontracted,
large-scale scalar relativistic MRCISD wavefunctions or based on the
scalar-relativistic solutions of the linear-response-theory-based
multi-configurational averaged quadratic coupled cluster method (LRT-MRAQCC).
The latter approach allows for a consistent, approximatively size-consistent
and size-extensive treatment of spin-orbit coupling. The approach is described
in detail and compared to a number of related techniques. The inherent accuracy
of the QDPT approach is validated by comparing cuts of the potential energy
surfaces of acrolein and its S, Se, and Te analoga with the corresponding data
obtained from matching fully variational spin-orbit MRCISD calculations. The
conceptual availability of approximate analytic gradients with respect to
geometrical displacements is an attractive feature of the 2c-QDPT-MRCISD and
2c-QDPT-LRT-MRAQCC methods for structure optimization and ab inito molecular
dynamics simulations.
