Quantum-Centric Computational Study of Methylene Singlet and Triplet States

This study involves quantum simulations of the dissociation of the ground-state triplet and first excited singlet states of the CH₂ molecule (methylene), which are relevant for interstellar and combustion chemistry. These were modeled as (6e, 23o) systems using 52 qubits on a quantum processor by applying the sample-based quantum diagonalization (SQD) method within a quantum-centric supercomputing framework. We evaluated the ability of SQD to provide accurate results of the singlet-triplet gap in comparison to selected configuration interaction (SCI) calculations and experimental values. To our knowledge, this is the first study of an open-shell system (the CH₂ triplet) using SQD. To obtain accurate energy values, we implemented post-SQD orbital optimization and employed a warm-start approach using previously converged states. The results for the singlet state dissociation were highly accurate, differing by only a few milli-Hartrees from the SCI reference values. Similarly, the SQD-calculated singlet-triplet energy gap aligned well with both experimental and SCI values, underscoring the method's capability to capture key features of CH₂ chemistry. However, the triplet state exhibited greater variability, likely due to differences in bit-string handling within the SQD method for open- versus closed-shell systems and the inherently complex wavefunction character of the triplet state. These findings highlight the strengths and limitations of SQD for modeling open-shell systems while laying a foundation for its application in large-scale electronic structure studies using quantum algorithms.