  Decoherence and gate errors severely limit the capabilities of
state-of-the-art quantum computers. This work introduces a strategy for
reference-state error mitigation (REM) of quantum chemistry that can be
straightforwardly implemented on current and near-term devices. REM can be
applied alongside existing mitigation procedures, while requiring minimal
post-processing and only one or no additional measurements. The approach is
agnostic to the underlying quantum mechanical ansatz and is designed for the
variational quantum eigensolver (VQE). Two orders-of-magnitude improvement in
the computational accuracy of ground state energies of small molecules (H2,
HeH+ and LiH) is demonstrated on superconducting quantum hardware. Simulations
of noisy circuits with a depth exceeding 1000 two-qubit gates are used to argue
for scalability of the method.
