Comparison of raw VQE (purple, ‘R’), PS-VQE (yellow, ‘PS’) and EV (green, ‘EV’) on an optimized unitary pair coupled-cluster ansatz. Darker (lighter) points correspond to six-qubit (6Q) (ten-qubit (10Q)) simulations. Error bars denote 1 s.d. calculated by either propagating error (VQE) or bootstrapping (EV). From left to right, the reaction path corresponds to the ring opening reaction. For the ten-orbital case, the unitary pair coupled-cluster ansatz (evaluated in simulation) has less than 1.8 x 10 -4 energy difference from exact diagonalization in the seniority-zero space. The blue curves correspond to the exact diagonalization of the seniority-zero active space Hamiltonian spanning ten orbitals (broad, lighter-blue line) and six orbitals (narrow, darker-blue line). The red curve is the RHF mean-field energy. Some of the data are plotted on a discontinuous and different scale to preserve the visual scale of the reaction energy along the reaction coordinate.
