Block correlated coupled cluster method with the complete active-space self-consistent-field reference function:...

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Accurate prediction of vertical excitation energies is crucial for understanding molecular electronic structure.
• Established methods like MR-CISD, CASPT2, and CC3 have limitations in accuracy and computational cost.
• Investigating novel computational methods is essential for advancing theoretical chemistry.

Purpose of the Study:

• To apply the Block Correlated Coupled Cluster (BCCC) method, specifically CAS-BCCC, to calculate vertical excitation energies.
• To evaluate the performance of the approximate CAS-BCCC4 method against established computational techniques.
• To assess the accuracy of CAS-BCCC4 for low-lying valence excited states in various organic molecules.

Main Methods:

• Utilized the Complete Active-Space Self-Consistent-Field (CAS-SCF) reference function.
• Employed an approximate CAS-BCCC with the cluster operator truncated to the four-block correlation level (CAS-BCCC4).
• Compared CAS-BCCC4 results with Multireference Configuration Interaction with Singles and Doubles (MR-CISD), MR-CISD+Q, Complete Active Space with Second-Order Perturbation Theory (CASPT2), and CC3.

Main Results:

• CAS-BCCC4 demonstrated competitive performance compared to multistate CASPT2 and outperformed single-state CASPT2 and MR-CISD.
• The method's accuracy was slightly inferior to MR-CISD+Q.
• Predictions for triplet excited states were consistent across various methods, while singlet excited states showed greater variability.

Conclusions:

• CAS-BCCC4 is a promising computational method for determining vertical excitation energies in organic molecules.
• The method offers a viable alternative to existing techniques, particularly for complex electronic systems.
• Further studies are warranted to refine the CAS-BCCC approach and expand its applicability.