Prediction of the singlet-triplet excitation energy for the spinel, MgTi₂O₄ via downfolding approach combined with first-principles Quantum Monte Carlo

The spinel, MgTi₂O₄ undergoes a transition into a dimerized state at low temperatures that is expected to be a spin singlet. However, no signature of a singlet-triplet transition as been discovered, in part due to the difficulty of predicting the energy of the transition from theory. Using high-accuracy first-principles quantum Monte Carlo combined with a novel model-fitting approach, we predict the singlet-triplet gap to be 350(50) meV, a higher energy than previous experimental observations have considered. Confirmation of our prediction would suggest that our approach should enable calculation of other excitation energies on the basis of first-principles quantum Monte Carlo combined with effective model calculations.