Reducing CAS-SDCI space. Using selected spaces in configuration interaction calculations in an efficient way.

Area of Science:

• Quantum chemistry
• Computational physics
• Theoretical chemistry

Background:

• Configuration Interaction (CI) methods are crucial for accurate electronic structure calculations.
• Large CI matrices pose significant computational challenges, limiting their application.
• Complete Active Space (CAS) methods provide a reference but can still be computationally intensive.

Purpose of the Study:

• To develop a novel method for reducing the size of CI matrices.
• To improve the computational efficiency of multi-reference internally contracted configuration interaction (MR-CCI) calculations.
• To enable more accurate and feasible electronic structure calculations for complex molecular systems.

Main Methods:

• A new method is presented to reduce CI matrix size by eliminating low-weight configurations from a CAS.
• Excited configurations are selectively added to the reference space, reducing the MR-SDCI space proportionally.
• A novel addressing technique drastically reduces computational time, matching CAS addressing efficiency.

Main Results:

• The method achieves a significant reduction in CI matrix size compared to full CAS-SDCI.
• Test calculations on N(2) and HCCH molecules demonstrate the accuracy of the reduced MR-SDCI space.
• Computational time is drastically reduced due to efficient addressing of the selected active space.

Conclusions:

• The developed method effectively reduces CI matrix size while maintaining accuracy.
• The approach is suitable for size-extensivity error correction techniques.
• This method offers a computationally efficient pathway for high-accuracy electronic structure calculations.