A dynamic correlation dressed complete active space method: Theory, implementation, and preliminary applications

Abstract

Complete Active Space SCF (CASSCF) theory may provide poor 0th order descriptions due to the lack of dynamic correlation. The most popular post-CASSCF approaches for recovering dynamic correlation are methods which keep the configuration interaction coefficients fixed at the CASSCF level and use internal contraction. This may result in severe inaccuracies where the wavefunction changes considerably under the influence of dynamic correlation. In this paper, we propose and compare several variants of a straightforward method of the "perturb-then-diagonalize" type that is aimed at keeping this balance while remaining computationally tractable and numerically stable. The method is loosely based on the theory of intermediate Hamiltonians and has been given the acronym "dynamic correlation dressed CAS" (DCD-CAS), with the second-order treatment, DCD-CAS(2), being the most practically useful member of the family. The dynamic correlation energy is treated to second order with a 0th order Hamiltonian based on Dyall's Hamiltonian. The method is orbitally invariant with respect to unitary transformations in the occupied, active, and virtual subspaces. It yields the ground- and low-lying excited states at the same time. Detailed numerical evaluations show that DCD-CAS(2) is superior to NEVPT2 for the difficult situations mentioned above while being very close to it when CASSCF provides a good 0th order description.

Structural, magnetic, redox and theoretical characterization of seven-coordinate first-row transition metal complexes with a macrocyclic ligand containing two benzimidazolyl N-pendant arms

“…Moreover, the recently introduced dynamic correlation dressed CAS with the second-order treatment (DCD-CAS(2)) was also utilized to calculate the ZFS parameters, where the spin-orbit and the spin-spin interactions were included. 69 VESTA 3 program was used to visualize the results of the calculations. 70…”

The Adsorption of Small Molecules on the Copper Paddle-Wheel: Influence of the Multi-Reference Ground State

“…On the side of ab initio methods we looked separately on single- and multi-reference approaches. Among the multi-reference ones, we used CASSCF (that one might expect to underrate the binding energy), NEVPT2 [ 73 , 74 , 75 ] (as a method superior to CASPT2) and quite novel DCD–CAS(2) [ 76 ]. Single-reference approaches are represented by SOS–MP2 [ 77 ] that is known to slightly overestimate non-covalent interactions in comparison to canonical MP2 [ 4 ] and DLPNO–CCSD without and with triples correction.…”

Effect of Coordination Geometry on Magnetic Properties in a Series of Cobalt(II) Complexes and Structural Transformation in Mother Liquor

“…48,49 Moreover, recently introduced dynamic correlation dressed CAS with a second-order treatment (DCD-CAS(2)) was also utilized to calculate ZFS parameters as well, where the spin−orbit and the spin−spin interactions were included. 50 ■ RESULTS AND DISCUSSION Synthesis. Complexes 1 and 3 were prepared by the reaction of Co(NCS) 2 and the bbp ligand in a mixture of DCM and MeOH (Scheme 2).…”