The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calcu- lations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of dia- batization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully vari- ational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on stan- dard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview.
Copyright © 2020, Journal of Chemical Physics. This article first appeared in Journal of Chemical Physics 152, no. 13 (April 7, 2020): 134110.
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Lischka, Hans, Ron Shepard, Thomas Mueller, Peter G. Szalay, Russell M. Pitzer, Adelia J. A. Aquino, Carol A. Parish, et al. “The Generality of the GUGA MRCI Approach in COLUMBUS for Treating Complex Quantum Chemistry.” Journal of Chemical Physics 152, no. 13 (April 7, 2020): 134110. https://doi.org/10.1063/1.5144267.