  In this article we present a machine learning model to obtain fast and
accurate estimates of the molecular Hessian matrix. In this model, based on a
random forest, the second derivatives of the energy with respect to redundant
internal coordinates are learned individually. The internal coordinates
together with their specific representation guarantee rotational and
translational invariance. The model is trained on a subset of the QM7 data set,
but is shown to be applicable to larger molecules picked from the QM9 data set.
From the predicted Hessian it is also possible to obtain reasonable estimates
of the vibrational frequencies, normal modes and zero point energies of the
molecules.
