Open-shell frozen natural orbital approach for quantum eigensolvers

Area of Science:

• Quantum Chemistry
• Computational Chemistry
• Theoretical Chemistry

Background:

• Accurate quantum chemical modeling of open-shell systems is crucial for understanding molecular properties and reactions.
• Traditional methods often struggle with large virtual spaces, limiting accuracy and computational efficiency.
• Developing resource-efficient methods is essential for tackling complex chemical problems.

Purpose of the Study:

• To introduce and validate a novel ZAPT2 frozen natural orbital (ZAPT-FNO) approach for open-shell systems.
• To demonstrate the efficiency and accuracy of ZAPT-FNO in reducing virtual space size while maintaining controllable accuracy.
• To enable accurate prediction of singlet-triplet energy gaps (T1-S0) for challenging molecular systems.

Main Methods:

• Implementation of the ZAPT2 perturbation theory for frozen natural orbital (FNO) selection in open-shell systems.
• Comparison of ZAPT-FNO with canonical molecular orbital truncation schemes.
• Application of ZAPT-FNO in conjunction with complete active space configuration interaction (CASCI) and iterative qubit coupled cluster (iQCC) methods.
• Simulation of T1-S0 gaps in H2O2, O2, stretched CH2, and the Ir(ppy)3 complex.

Main Results:

• The ZAPT-FNO scheme significantly outperforms standard virtual space truncation methods, especially with large basis sets.
• ZAPT-FNO-selected virtual orbitals lead to systematic convergence of correlation energies and T1-S0 gaps.
• Accurate T1-S0 gaps were obtained for H2O2, O2, and stretched CH2, consistent with CASCI and iQCC results.
• Application to the Ir(ppy)3 complex demonstrated robust convergence and near-experimental accuracy for T1-S0 gaps.

Conclusions:

• The ZAPT-FNO method provides a resource-efficient pathway to high-accuracy quantum chemical modeling of open-shell states.
• This approach enables the simulation of large materials with extended basis sets and realistic active space sizes.
• ZAPT-FNO significantly enhances the capability of computational chemistry for studying complex open-shell systems.