High-level multireference methods in the quantum-chemistry program system COLUMBUS: Analytic MR-CISD and MR-AQCC gradients and MR-AQCC-LRT for excited states, GUGA spin-orbit CI and parallel CI density

Development of several new computational approaches within the framework of multi-reference ab initio molecular electronic structure methodology and t heir implementation in the COLUMBUS program system are reported. These new features are: calculation of the analytical MR-CI gradient for excited stat es based on state-averaged MCSCF orbitals, the extension of the MR-ACPF/AQC C methods to excited states in the framework of linear-response theory, spi n-orbit CI for molecules containing heavy atoms and the development of a ma ssively-parallel code for the computation of the one- and two-particle dens ity matrix elements. Illustrative examples are given for each of these case s.