Cascaded variational quantum eigensolver calculations of molecular and ionic compounds*

We present results of many-electron ground-state calculations of molecular and ionic compounds obtained using the cascaded variational quantum eigensolver (CVQE) algorithm. We focus on molecules and ions that are involved in chemical reactions that contribute to aqueous protonic diffusion, which is one of many mechanisms involved in corrosion. This includes the hydronium cation H₃O⁺, which has 8 valence electrons occupying many-electron states constructed from 14 valence spin-orbitals.  The results include CVQE data obtained using quantum simulators as well as IonQ quantum hardware. The IonQ Aria QPU has 25 algorithmic qubits with excellent gate fidelities and all-to-all qubit connectivity, which offers a great deal of flexibility to optimize the CVQE circuits by reducing the number of two-qubit entangling gates.  Pairing the optimized circuit with error detection techniques, we execute the CVQE quantum circuits on the Aria QPU to calculate the many-electron ground-state energy of the hydronium cation.