The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry

Abstract

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 calculations (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 -conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational 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 standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy (ACME) multiconfiguration self-consistent field (MCSCF) 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, the development of

Conformer-Specific Dissociation Dynamics in Dimethyl Methylphosphonate Radical Cation

“…We had to reduce the average of states from 7 to 5 and remove the diffuse functions because it was difficult to converge the calculations. The COLUMBUS software was used for the optimizations [ 50 , 51 , 52 ]. Because of the flexibility of the molecule it was not always possible to converge at the minimum of the seam.…”

“…All CCSD calculations were done using Q-Chem suite of packages [ 53 ], while CASSCF calculations were performed using MOLPRO suite of packages [ 54 ]. The conical intersection searches were performed using COLUMBUS [ 50 , 51 , 52 ].…”

A multiconfigurational pair-density functional theory approach to molecular junctions

“…A significant number of previous works have focused on schemes for active space selection in order to obtain better reaction energetics or other properties of molecular systems. [71][72][73][74][75][76][77][78][79][80] Simulations of molecular junctions using a CASSCF approach may require alternative active space selection schemes which are different than the better-studied molecular-based schemes. For the benzyne junction, we employed active spaces that ranged in size from (6e,6o) to (14e,14o), and we also explored two different active orbital selection schemes.…”

Machine learning for electronically excited states of molecules

“…Another variant of MCSCF methods is restricted active space SCF (RASSCF), which is very similar to CASSCF, but within RASSCF the active space is restricted and no FCI computation is carried out. 256 MR-CI Even higher accuracy can be obtained with multi-reference CI methods, 29,274,275 such as MR-CISD, that additionally add single and double excitations out of the active space and are therefore based on CASSCF wave functions. With this approach electronic correlation, i.e.…”