Novel strategy to implement active-space coupled-cluster methods

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Coupled-cluster (CC) methods are essential for accurate electronic structure calculations.
• Implementing CC methods with higher excitations remains computationally demanding.
• Efficient algorithms are crucial for advancing theoretical chemistry.

Purpose of the Study:

• To develop a novel, efficient computational framework for coupled-cluster (CC) methods.
• To enable accurate calculations including higher excitation levels.
• To automate the implementation of multi-reference CC equations.

Main Methods:

• A hybrid approach utilizing string representations and matrix products for amplitude and intermediate contractions.
• A new diagrammatic technique for compact representation of CC equations.
• Automated implementation of single-reference-based multi-reference CC equations for arbitrary excitation levels.

Main Results:

• The new method demonstrates efficient evaluation of diagram contributions via vectorized subroutines.
• Timing results for test calculations on complete active-space problems show performance gains.
• The code was successfully applied to study weak interactions in the Be dimer.

Conclusions:

• The presented approach offers a significant improvement in the efficiency and applicability of CC methods.
• The automated implementation facilitates complex quantum chemical calculations.
• This work advances the capability to study intricate molecular systems and interactions.