A multistate local coupled cluster CC2 response method based on the Laplace transform.

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Theoretical Chemistry

Background:

• Calculating excitation energies for extended molecular systems is computationally demanding.
• Previous local coupled cluster doubles (CC2) response methods faced challenges in accuracy and robustness for certain excited states.

Purpose of the Study:

• To introduce a novel Laplace transform-based multistate local CC2 response method.
• To enhance the efficiency and reliability of calculating excitation energies for large molecular systems.

Main Methods:

• Developed a Laplace transform technique to partition the CC2 Jacobian, preserving sparsity.
• Implemented a multistate treatment with adaptive local approximations for excited states.
• Integrated density fitting for two-electron integrals and proposed a novel procedure for specifying local approximations.

Main Results:

• The new Laplace transformed density fitted local CC2 (LT-DF-LCC2) response method effectively calculates excitation energies.
• The method demonstrates improved robustness compared to earlier local CC2 response methods, successfully identifying excited states in challenging cases.
• Performance and accuracy were validated across various test molecules and electronic states.

Conclusions:

• The LT-DF-LCC2 response method offers a significant advancement for computing excitation energies in extended molecular systems.
• This approach provides a more reliable and robust computational tool for theoretical and computational chemists.
• The method's ability to handle difficult cases marks a substantial improvement in the field.