The analysis of a chemical reaction along the ground-state
potential
energy surface in conjunction with an unknown spin state is challenging
because electronic states must be separately computed several times
using different spin multiplicities to find the lowest energy state.
However, in principle, the ground state could be obtained with just
a single calculation using a quantum computer without specifying the
spin multiplicity in advance. In the present work, ground-state potential
energy curves for PtCO were calculated as a proof-of-concept using
a variational quantum eigensolver (VQE) algorithm. This system exhibits
a singlet–triplet crossover as a consequence of the interaction
between Pt and CO. VQE calculations using a statevector simulator
were found to converge to a singlet state in the bonding region, while
a triplet state was obtained at the dissociation limit. Calculations
performed using an actual quantum device provided potential energies
within ±2 kcal/mol of the simulated energies after error mitigation
techniques were adopted. The spin multiplicities in the bonding and
dissociation regions could be clearly distinguished even in the case
of a small number of shots. The results of this study suggest that
quantum computing can be a powerful tool for the analysis of the chemical
reactions of systems for which the spin multiplicity of the ground
state and variations in this parameter are not known in advance.