Nonunitary variational quantum eigensolver with the localized active space method and cost mitigation

The Journal of Physical Chemistry A (2025)

 

One of the big challenges for quantum chemistry is accurately describing molecules where electrons are strongly interacting while keeping the calculations affordable on today’s small, noisy quantum computers. A method called LASSCF breaks a molecule into fragments, treating each piece carefully while handling the interactions between pieces in a simpler way, making it less costly than traditional approaches.

In this paper, scientists combine LASSCF with a modified version of the variational quantum eigensolver (VQE), called nuVQE, which adds special qubit operators. These operators capture electron correlations without making the quantum circuits deeper, which is important for current hardware. The combined method — LAS-nuVQE — achieves chemical accuracy for test molecules using fewer than 70 quantum gates, outperforming standard approaches.

The approach shows that accurate quantum chemistry simulations of complex systems are possible on today’s limited quantum devices by carefully combining classical approximations with efficient quantum algorithms.

The paper is authored by Qiaohong Wang, Ruhee D’Cunha, Abishek Mitra, Stephen Gray, Matt Otten and Laura Gagliardi, who conducted the work at the University of Chicago, Argonne National Laboratory and the University of Wisconsin–Madison.