Quantum Chemistry Breakthrough: Simulating Radical Molecules with Quantum Processors

IBM Quantum Chemistry Simulation

In a groundbreaking study published in the Journal of Chemical Theory and Computation, scientists from IBM Quantum® and Lockheed Martin have demonstrated that quantum computers can accurately simulate open-shell molecules—a long-standing challenge for classical computational chemistry. Their subject: the deceptively simple but chemically complex radical species known as methylene (CH₂).

This marks the first application of the Sample-based Quantum Diagonalization (SQD) technique to an open-shell system, establishing a new benchmark for quantum advantage in computational chemistry. Open-shell molecules contain unpaired electrons, which give rise to intricate quantum behavior, magnetic properties, and high reactivity—attributes that are notoriously difficult to simulate on classical machines.

Quantum Chemistry and the Case for SQD

Traditional high-performance computing methods often struggle to model systems with strong electron correlation—such as transition states, radicals, and excited states—because the computational complexity grows exponentially with the number of interacting electrons. Quantum computers, on the other hand, can encode and process entangled quantum states directly, making them ideally suited to tackle these systems.

In this work, the IBM and Lockheed Martin team used SQD to simulate CH₂'s singlet and triplet states, successfully computing dissociation energies, electronic transitions, and the singlet-triplet energy gap. These simulations were executed on a 52-qubit IBM quantum processor using up to 3,000 two-qubit gates per experiment, all within IBM’s quantum-centric supercomputing architecture that blends classical and quantum computing resources.

Why CH₂ Matters

Despite consisting of just three atoms, CH₂ plays a major role in combustion chemistry, atmospheric reactions, and interstellar molecular dynamics. In its ground state, CH₂ is a triplet diradical—a rare configuration where two unpaired electrons exist with parallel spins. This makes CH₂ both highly reactive and challenging to model accurately.

The singlet state of CH₂, known as a carbene, is even trickier to simulate due to complex electron correlations. Accurate prediction of the energy difference between these two states—the singlet-triplet gap—is critical for understanding CH₂’s reactivity in real-world environments such as combustion engines or sensor systems.

Key Outcomes of the Quantum Simulations

  • 📈 Singlet dissociation energy computed within a few milliHartrees of high-accuracy classical benchmarks (SCI).
  • ⚛️ Triplet energies matched experimental data near equilibrium geometries.
  • 🔍 Accurate singlet-triplet gap aligned with both experimental and classical values.

This study is not just a proof-of-concept. It provides real chemical insight into bond dissociation dynamics, transition states, and radical behavior in CH₂—paving the way for future applications in aerospace, materials science, and environmental monitoring.

Why This Breakthrough Matters

Open-shell systems are central to many advanced technologies, including:

  • 🔥 Combustion modeling and propulsion systems
  • 🧪 Catalytic cycle analysis
  • 📡 Design of novel sensor materials for trace species detection

By enabling accurate quantum simulations of such systems, this work opens the door to predictive materials design and next-generation chemical engineering powered by quantum computing.

🔗 Original article citation: IBM Quantum Blog – Lockheed Martin and IBM apply quantum computing to simulate methylene (May 2025)

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