An efficient perturbational treatment of spin-orbit coupling within theframework of high-level multi-reference techniques has been implemented in themost recent version of the COLUMBUS quantum chemistry package, extending theexisting fully variational two-component (2c) multi-reference configurationinteraction singles and doubles (MRCISD) method. The proposed scheme followsrelated implementations of quasi-degenerate perturbation theory (QDPT) modelspace techniques. Our model space is built either from uncontracted,large-scale scalar relativistic MRCISD wavefunctions or based on thescalar-relativistic solutions of the linear-response-theory-basedmulti-configurational averaged quadratic coupled cluster method (LRT-MRAQCC).The latter approach allows for a consistent, approximatively size-consistentand size-extensive treatment of spin-orbit coupling. The approach is describedin detail and compared to a number of related techniques. The inherent accuracyof the QDPT approach is validated by comparing cuts of the potential energysurfaces of acrolein and its S, Se, and Te analoga with the corresponding dataobtained from matching fully variational spin-orbit MRCISD calculations. Theconceptual availability of approximate analytic gradients with respect togeometrical displacements is an attractive feature of the 2c-QDPT-MRCISD and2c-QDPT-LRT-MRAQCC methods for structure optimization and ab inito moleculardynamics simulations.
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