title: Perturbational treatment of spin-orbit coupling for generally applicable high-level multi-reference methods.
abstract: 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.
authors.affiliation: Institute of Theoretical Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Mai
authors.affiliation: Institute for Advanced Simulation, Jülich Supercomputing Centre, Forschungszentrum Jülich, 52425 Jülich, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Müller
authors.affiliation: Interdisciplinary Center for Scientific Computing, University of Heidelberg, Im Neuenheimer Feld 368, 69120 Heidelberg, Germany.
authors.forename: Felix
authors.initials: F
authors.lastname: Plasser
authors.affiliation: Institute of Theoretical Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Philipp
authors.initials: P
authors.lastname: Marquetand
authors.affiliation: Institute of Theoretical Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Hans
authors.initials: H
authors.lastname: Lischka
authors.affiliation: Institute of Theoretical Chemistry, University of Vienna, Währinger Str. 17, 1090 Vienna, Austria.
authors.forename: Leticia
authors.initials: L
authors.lastname: González