Computational Analysis of Chemical Reactions Using a Variational Quantum Eigensolver Algorithm without Specifying Spin Multiplicity

The analysis of a chemical reaction along the ground-statepotentialenergy surface in conjunction with an unknown spin state is challengingbecause electronic states must be separately computed several timesusing different spin multiplicities to find the lowest energy state.However, in principle, the ground state could be obtained with justa single calculation using a quantum computer without specifying thespin multiplicity in advance. In the present work, ground-state potentialenergy curves for PtCO were calculated as a proof-of-concept usinga variational quantum eigensolver (VQE) algorithm. This system exhibitsa singlet-triplet crossover as a consequence of the interactionbetween Pt and CO. VQE calculations using a statevector simulatorwere found to converge to a singlet state in the bonding region, whilea triplet state was obtained at the dissociation limit. Calculationsperformed using an actual quantum device provided potential energieswithin +/- 2 kcal/mol of the simulated energies after error mitigationtechniques were adopted. The spin multiplicities in the bonding anddissociation regions could be clearly distinguished even in the caseof a small number of shots. The results of this study suggest thatquantum computing can be a powerful tool for the analysis of the chemicalreactions of systems for which the spin multiplicity of the groundstate and variations in this parameter are not known in advance.