Dynamically weighted multireference perturbation theory: Combining the advantages of multi-state and state-averaged...

Area of Science:

• Quantum chemistry
• Computational physics
• Electronic structure theory

Background:

• Calculating near-degenerate electronic states is crucial for understanding molecular properties.
• Existing methods like state-averaged and state-specific approaches have limitations in accurately describing complex electronic structures.

Purpose of the Study:

• Introduce two novel computational approaches based on the driven similarity renormalization group (DSRG) framework.
• Develop methods to accurately compute near-degenerate electronic states, including avoided crossings and conical intersections.

Main Methods:

• Formulated a unitary multi-state formalism based on DSRG (MS-DSRG).
• Developed dynamically weighted DSRG (DW-DSRG) theory, interpolating between multi-state and state-averaged DSRG.
• Applied second-order perturbation theory (PT2) to both MS-DSRG and DW-DSRG.

Main Results:

• DW-DSRG-PT2 accurately predicted the avoided crossing of LiF, avoiding artifacts of other methods.
• Computed excitation energies for acenes (benzene, naphthalene, anthracene) using DW-DSRG-PT2 showed improved accuracy over state-averaged DSRG.
• Demonstrated that DW-DSRG results exhibit minimal dependence on the number of states included in the calculation.

Conclusions:

• The developed DW-DSRG-PT2 method offers a robust approach for calculating electronic states, particularly near degeneracies.
• DW-DSRG effectively combines the strengths of multi-state and state-averaged methods, providing accurate descriptions of molecular electronic structures.
• These advancements offer improved computational tools for studying excited states and reaction pathways in molecules.