Analytic gradients for EOM-DEA-CCSD and EOM-DIP-CCSD: Theory, implementation, and application to diradicals.

Area of Science:

• Quantum Chemistry
• Computational Chemistry

Background:

• Equation-of-motion coupled-cluster theory with single and double excitations (EOM-CCSD) is a powerful tool for electronic structure calculations.
• Specific formulations like EOM-DEA-CCSD and EOM-DIP-CCSD excel at describing open-shell species, including diradicals.

Purpose of the Study:

• To develop and implement analytic nuclear gradients for EOM-DEA-CCSD and EOM-DIP-CCSD.
• To enhance the capability for accurate geometry optimizations and property calculations of open-shell systems.

Main Methods:

• Derivation and implementation of analytic nuclear gradients for EOM-DEA-CCSD and EOM-DIP-CCSD.
• Extension of the framework to include spin-orbit coupling for EOM-DEA-CCSD states.

Main Results:

• Successful implementation of analytic nuclear gradients for EOM-DEA-CCSD and EOM-DIP-CCSD.
• Calculations of singlet-triplet gaps in benzyne diradicals.
• Characterization of molecules relevant to quantum information science.
• Inclusion of spin-orbit coupling for EOM-DEA-CCSD states, enabling intensity borrowing and intersystem crossing calculations.

Conclusions:

• The developed analytic gradients significantly improve the accuracy of computational studies for open-shell systems.
• These advancements facilitate detailed investigations of diradicals and molecules crucial for quantum information science applications.
• The inclusion of spin-orbit coupling broadens the scope of EOM-CCSD methods for complex electronic structure problems.