Equation-of-motion coupled-cluster method with double electron-attaching operators: Theory, implementation, and...

Area of Science:

• Quantum chemistry
• Computational physics
• Theoretical chemistry

Background:

• The equation-of-motion coupled-cluster (EOM-CC) framework is a powerful tool for electronic structure calculations.
• Accurately describing systems with multi-reference character, such as diradicals and bond-breaking processes, remains a challenge for standard EOM-CC methods.

Purpose of the Study:

• To implement and evaluate a production-level equation-of-motion coupled-cluster method with double electron-attaching (DEA) EOM operators.
• To assess the performance of the newly developed EOM-DEA-CCSD method for calculating energy differences and molecular properties.
• To compare the accuracy of EOM-DEA-CCSD wave functions with those from other EOM-CC methods using reduced density matrices.

Main Methods:

• Implementation of the equation-of-motion coupled-cluster singles and doubles (EOM-CCSD) method incorporating double electron-attaching (DEA) EOM operators.
• Development of 2p and 3p1h type DEA EOM operators within the EOM-CC formalism.
• Analysis of electronic structure patterns suitable for the 'two-electrons-in-many orbitals' ansatz.
• Computation of energy differences and molecular properties using EOM-DEA-CCSD.
• Comparison of EOM-DEA-CCSD wave functions with other EOM-CCSD methods via state and transition one-particle density matrices.

Main Results:

• Successful production-level implementation of the EOM-DEA-CCSD method.
• Demonstrated capability of EOM-DEA-CCSD to accurately describe challenging electronic structure phenomena.
• Benchmarks show EOM-DEA-CCSD's effectiveness in treating diradicals, bond-breaking processes, and certain conical intersections.
• Analysis of one-particle density matrices confirms the validity and accuracy of the EOM-DEA-CCSD wave functions.

Conclusions:

• EOM-DEA-CCSD is a valuable addition to the EOM-CC family of methods.
• The method provides an accurate and efficient way to study systems with multi-reference character.
• EOM-DEA-CCSD offers a robust approach for investigating complex molecular behaviors in quantum chemistry.