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

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Complete Active Space Self-Consistent Field (CASSCF) theory can yield inaccurate 0th order descriptions due to missing dynamic correlation.
• Existing post-CASSCF methods may struggle when wavefunctions significantly change under dynamic correlation effects.

Purpose of the Study:

• To introduce and evaluate novel
• perturb-then-diagonalize
• methods for incorporating dynamic correlation into CASSCF calculations.
• To present the dynamic correlation dressed CAS (DCD-CAS) method, particularly its second-order variant (DCD-CAS(2)).

Main Methods:

• Development of the DCD-CAS method, based on intermediate Hamiltonians theory.
• Application of a second-order perturbation treatment for dynamic correlation energy using Dyall's Hamiltonian.
• Ensuring orbital invariance with respect to unitary transformations in occupied, active, and virtual subspaces.

Main Results:

• DCD-CAS(2) is computationally tractable and numerically stable.
• The method simultaneously yields ground and low-lying excited states.
• DCD-CAS(2) outperforms NEVPT2 in challenging cases where CASSCF is a poor 0th order approximation.

Conclusions:

• DCD-CAS(2) provides a superior and accurate approach for including dynamic correlation compared to NEVPT2 in specific scenarios.
• The method remains competitive with NEVPT2 when the initial CASSCF description is already good.
• DCD-CAS offers a robust and versatile tool for electronic structure calculations in quantum chemistry.