Prediction for the singlet-triplet excitation energy for the spinel MgTi2 O4 using first-principles diffusion 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 has been discovered, in part due to the difficulty of predicting the energy of the transition from theory. We find that the dimers of MgTi2O4 can be described by a Heisenberg model with very small interactions between different dimers. Using high-accuracy first-principles quantum Monte Carlo combined with a model-fitting approach, we predict the singlet-triplet gap of these dimers to be 350(50) meV, a higher energy than previous experimental observations have considered. The prediction is published in advance of experimental confirmation.