Researchers at IBM and Lockheed Martin teamed up high-performance computing with quantum computing to accurately model the electronic structure of ‘open-shell’ molecules, methylene, which has been a hurdle with classic computing over the years. This is the first demonstration of the sample-based quantum diagonalization (SQD) technique to open-shell systems, a press release said.
Quantum computing, which promises computations at speeds unimaginable by even the fastest supercomputers of today, is the next frontier of computing. Leveraging quantum states of molecules to serve as quantum bits, these computers supersede computational capabilities that humanity has had access to in the past and open up new research areas.
Quantum chemistry is one such area of interest where understanding the interaction between molecules can help design more efficient processes for industrial applications as well as material research. However, chemical systems involving strong electron correlation have been exceptionally hard to simulate, even on high-performance computing platforms.
The challenge with open-shell systems
Classical approximation methods used for simulating chemical systems with strong electronic correlation exist. However, computational costs associated with systems increase exponentially with the number of electrons involved.
Understanding energy states and how molecules transition from one state to another is crucial to our understanding of chemistry. Doing so allows scientists to predict their reactivity and how they will interact with catalysts or leverage their excited state to carry out sensing or aerospace applications and much more.
The challenge of these simulations is even higher with ‘open-shell systems’ where molecules contain one or more unpaired electrons. Unlike their closed-shell counterparts, whose electrons are paired in orbitals and have simple and stable wave functions, open-shell molecules are highly reactive, can exhibit magnetic properties, and need multiple wave functions to capture their complexity.
Researchers at IBM and Lockheed Martin were especially interested in understanding the reactivity of methylene, one such open-shell molecule, and examined it using a combination of high-performance and quantum computing.
Why did the team study methylene?
Methylene (CH2) is composed of three atoms but is quite complex in nature. In its ground state, the molecule adopts a triplet electronic structure where the carbon atom’s outer shell consists of two unpaired electrons with parallel spins. This is a highly energetic state, and the molecule is not generally found in it.
The molecule has a first excited state called carbene singlet state where two electrons in the carbon atom are paired with opposite spins and one orbital is empty. In its triplet state, the two electrons are unpaired with spins in the same direction. This leads to a rare occurrence where the molecule’s triplet state is lower than its singlet state.
The energy difference between the singlet-triplet states is crucial since it allows scientists to predict how the molecule will interact in a chemical reaction. Using IBM’s quantum-centric supercomputing framework, combining high-performance computing with quantum processors, the researchers applied the Sample-based Quantum Diagonalization (SQD) method and computed methylene’s singlet and triplet states, their dissociation energies and energy gaps.
Molecules such as methylene are crucial for aerospace and combustion chemistry and help design better engines, while also helping demonstrate the present-day application of quantum computing.