A quantum computing breakthrough by researchers at IBM and Daimler AG, the parent company of Mercedes-Benz, uses a quantum computer to model the dipole moment of three lithium-containing molecules, which brings us one step closer the next-generation lithium sulfur (Li-S) batteries that would be more powerful, longer lasting and cheaper than today’s widely used lithium ion batteries.
Simulating molecules is extremely difficult but modeling them precisely is crucial to discover new drugs and materials. In the research paper “Quantum Chemistry Simulations of Dominant Products in Lithium-Sulfur Batteries,” we simulated 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 ever on quantum hardware, we demonstrated that we can calculate the dipole moment for LiH using 4 qubits on IBM Q Valencia, a premium-access 5-qubit quantum computer.
Quantum chemistry simulations of four industrially relevant molecules are reported. Dissociation curves and dipole moments are reported for lithium hydride (LiH), hydrogen sulfide (H2S), lithium hydrogen sulfide (LiSH) and lithium sulfide (Li2S). Herein, we demonstrate the ability to calculate dipole moments using up to 21 qubits on a quantum simulator for a lithium sulfur salt molecule, and demonstrate the ability to calculate the dipole moment of the LiH molecule on the IBM Q Valencia device using four qubits. This is the first example to the best of our knowledge of dipole moment calculations being performed on quantum hardware.
Quantum Computers for Faster Improvement of Chemistry and Technology
Researchers at Daimler hope that quantum computers will help them design next-generation lithium-sulfur batteries. They can compute and precisely simulate their fundamental behavior. Quantum computers are not yet better than classical computers. They are very ‘noisy,’ meaning that any outside disturbance knocks the fragile qubits out of quantum states crucial for the calculation too early for them to run meaningful computations. Still, they are already showing great promise in chemistry, towards precisely simulating complex molecules.
The main aim of molecular simulation, on any machine, is to find a compound’s ground state—its most stable configuration. This is no trivial task because it requires simulating the interactions between all the particles, such as electrons, in the molecule. And the bigger and more complex a molecule and its environment is, the more difficult this process becomes.
Today’s supercomputers can simulate fairly simple molecules, but when researchers try to develop novel, complex compounds for better batteries and life-saving drugs, traditional computers can no longer maintain the accuracy they have at smaller scales. The solution has typically been to model experimental observations from the lab and then test the theory.
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.