Efficient Implementation of the Spin-Free Renormalized Internally-Contracted Multireference Coupled Cluster Theory

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Multireference coupled cluster methods are crucial for accurately describing strongly correlated electronic systems.
• Existing implementations can be computationally expensive and challenging to scale for large molecules.

Purpose of the Study:

• To report an efficient implementation of the renormalized internally contracted multireference coupled cluster with singles and doubles (RIC-MRCCSD) method within the ORCA quantum chemistry program.
• To enhance the computational efficiency and scalability of RIC-MRCCSD for practical applications.

Main Methods:

• Integration of Evangelista's Wick&d equation generator with ORCA's AGE code generator to implement many-body residuals.
• Development of a spin-free formulation for RIC-MRCCSD to improve efficiency over spin-orbital versions.
• Leveraging AGE's parallelized code generation for multi-core execution and speedups.

Main Results:

• Achieved substantial efficiency gains through the spin-free formulation and parallelized code generation.
• Demonstrated computational cost between RHF-CCSD and UHF-CCSD, even for large active spaces (e.g., CAS(14,14)).
• Successfully scaled the method to a vitamin B12 model with CAS(12,12) and 809 orbitals, showcasing its applicability to large systems.

Conclusions:

• The implemented RIC-MRCCSD method offers a significant improvement in efficiency and scalability for quantum chemical calculations.
• The method's performance is competitive with other high-level correlated methods, providing accurate results without requiring high-order density matrices.
• Further investigation into parameters like 'flow' is needed to fully optimize the RIC-MRCCSD theory.