Researchers at IBM and Daimler AG have used a quantum computer to model the dipole moment of three lithium-containing molecules, with an eye on moving closer to next-generation lithium sulfur (Li-S) batteries.

In a research paper published on arXiv.org, the researchers report simulating the ground state energies and the dipole moments of the molecules that could form in lithium-sulfur batteries during operation: lithium hydride (LiH), hydrogen sulfide (H₂S), lithium hydrogen sulfide (LiSH), and the desired product, lithium sulfide (Li₂S). In addition, and for the first time on quantum hardware, they demonstrated that they can calculate the dipole moment for LiH using 4 qubits on IBM Q Valencia, a premium-access 5-qubit quantum computer.

In a post on the IBM Research Blog, co-author Jeannette Garcia at IBM Almaden Research Center notes that quantum computers are not yet better than classical computers. Any outside disturbance knocks the fragile qubits out of quantum states crucial for the calculation too early for them to run meaningful computations. However, she adds, they are already showing great promise in chemistry, towards precisely simulating complex molecules—a process that is time-consuming and costly on classic computers.

The largest chemical problems researchers have been so far able to simulate classically, meaning on a standard computer, by exact diagonalization (or FCI, full configuration interaction) comprise around 22 electrons and 22 orbitals, the size of an active space in the pentacene molecule. For reference, a single FCI iteration for pentacene takes ~1.17 hours on ~4096 processors and a full calculation would be expected to take around nine days. For any larger chemical problem, exact calculations become prohibitively slow and memory-consuming, so that approximation schemes need to be introduced in classical simulations, which are not guaranteed to be accurate and affordable for all chemical problems. It’s important to note that reasonably accurate approximations to classical FCI approaches also continue to evolve and is an active area of research, so we can expect that accurate approximations to classical FCI calculations will also continue to improve over time.

That’s where quantum computers come in. Qubits themselves operate according to the laws of quantum mechanics, just like the molecules researchers are trying to simulate. The hope is that in time quantum computers can greatly speed up the simulation process by precisely predicting the properties of a new molecule that can explain its behavior, such as reactivity. Programming qubits works by using unique properties of superposition and entanglement, allowing the potential for researchers to evaluate a expectation parameters—in a much more efficient way than a standard computer ever could.

—Jeannette Garcia

The researchers at Daimler hope that quantum computers will help them design next-generation lithium-sulfur batteries, because quantum computers have the potential to compute and simulate precisely the fundamental behavior.

The electron-cloud density distribution of molecules, and particularly their dipole moment, are critical for understanding a variety of phenomena occurring in batteries. In general, molecules with high polarity can easily attract or repel valence electrons from other compounds and generate reactions through electron transfer. The dipole moment of a molecule also determines its response to an external electric field. Accurate computation of energetics and dipole moments of molecules is thus a problem with deep conceptual importance, and significant applicability to the chemistry of LiS batteries. Achieving this goal requires solving the Schrödinger equation for the molecules of interest, a problem that is known to be exponentially expensive for classical computers, unless approximation schemes are introduced.

Quantum computing is a mode of attack of mathematical problems, that has an enormous potential to provide advantage over conventional computing in a number of areas, including quantum chemistry. A number of heuristics to provide approximate but highly accurate solutions to the Schrödinger equation have been proposed, in particular the Variational Quantum Eigensolver (VQE). IBM researchers have demonstrated the use and accuracy of VQE in investigations of a variety of molecules.

Motivated by these successes, and by the importance of computing energies and electrostatic properties, in this work we assess the performance of quantum algorithms in determining ground state energies and dipole moments along bond stretching for LiH, H₂S, LiSH and Li₂S.

—Rice et al.

To make sure the calculations on the quantum hardware were accurate, the researchers also performed them on a classical computer using the IBM quantum simulator. Then, they ran these calculations on IBM Q Valencia, and compared the results.

Resources

• Julia E. Rice, Tanvi P. Gujarati, Tyler Y. Takeshita, Joe Latone, Mario Motta, Andreas Hintennach, Jeannette M. Garcia (2020) “Quantum Chemistry Simulations of Dominant Products in Lithium-Sulfur Batteries” arXiv:2001.01120 [physics.chem-ph]