Second-Order Complete Active Space Perturbation Theory (CASPT2) and N-Electron Valence State Perturbation Theory...

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate electronic structure calculations are crucial for understanding molecular properties.
• High-level methods like CASPT2 and NEVPT2 often require significant computational resources, particularly memory.
• The four-particle reduced density matrix (4RDM) is a key component in these calculations but is memory-intensive to store.

Purpose of the Study:

• To develop and implement efficient CASPT2 and NEVPT2 calculations.
• To reduce the memory footprint associated with 4RDM storage in these methods.
• To demonstrate the applicability of the new approach to challenging chemical systems.

Main Methods:

• Development of CASPT2 and NEVPT2 methods.
• Utilizing an adaptive sampling configuration interaction self-consistent field (ASCI-SCF) reference function.
• Direct calculation of intermediate matrices to avoid explicit 4RDM storage.

Main Results:

• Successfully implemented memory-efficient CASPT2 and NEVPT2 calculations.
• Demonstrated the method's performance on polyacenes (singlet-triplet gaps) and chromium dimer (potential energy curves).
• Successfully handled large active spaces, up to (34e,34o).

Conclusions:

• The developed method offers a computationally efficient approach for high-level electronic structure calculations.
• This advancement significantly reduces memory requirements, enabling the study of larger and more complex systems.
• The method is validated by its successful application to relevant chemical problems.