A note on large-scale quantum chemistry on quantum computers: the case of a molecule with half-Möbius topology

TL;DR

It is demonstrated that a systematic increase of active space sizes is achievable with state-of-the-art quantum processors, thus offering a promising path towards practically relevant quantum-assisted electronic-structure calculations.

Abstract

We report quantum chemistry calculations performed on superconducting quantum processors for a molecule exhibiting the half-Möbius electronic topology originally introduced by Rončević et al. Using SqDRIFT, a randomized sample-based Krylov quantum diagonalization algorithm, we achieve reliable quantum simulations on active spaces corresponding to 36 orbitals (72 qubits) and extend previous studies up to 50 orbitals (100 qubits). We demonstrate that a systematic increase of active space sizes, which has a concrete impact on the accuracy of the electronic structure description, is achievable with state-of-the-art quantum processors, thus offering a promising path towards practically relevant quantum-assisted electronic-structure calculations.

Figures (3)

• Figure 1: The SqDRIFT algorithm. The molecular Hamiltonian is mapped to qubit operators. Its terms $h_k$ generate time-evolution unitaries $U_k$. Circuits are executed within a fixed depth budget (dashed region). In SKQD, deterministic sequences of $U_k$ are applied, while in SqDRIFT the unitaries are sampled according to their relative importance. Measurement samples are then collected and post-processed to obtain the final energies.
• Figure 2: Dyson orbital. Isosurface of the Dyson orbital computed with the SqDRIFT algorithm with $72$ qubits, corresponding to an active space of $32$ electrons in $36$ orbitals.
• Figure 3: Correlation energy (relative to Hartree–Fock) as a function of the active space size (number of spin orbitals/qubits) at a fixed number of sampled configurations for the diagonalization (subspace dimension $\bm{\mathrm{d}}\approx 10^7$). Blue points correspond to the energy evaluated with the (extended (Ext) Barison2025, red contours) SqDRIFT algorithm as described in Piccinelli2026. The triangle corresponds to the best energy value obtained with the data of Ref. Roncevic2026. The squares are instead obtained using improved hardware and extended to a growing number of spatial orbitals. For the classical calculations, the green dashed line shows the CASSCF results for $12$ electrons in $12$ orbitals and the red circles correspond to the energies computed with HCI with an equivalent number of samples as for the SqDRIFT calculations.