The MLFF are trained and applied using the VASP implementation , which is similar to the Gaussian Approximation Potential (GAP) . The radial and angular descriptors are constructed using the smooth overlap of atomic positions (SOAP) with cutoffs of 5 Å for both radial and angular parts. We follow the procedure for constructing accurate force fields ; first a force field is fitted using Bayesian linear regression (BLR) with an atomic broadening of 0.3 Å and 12 radial basis functions used to expand the radial descriptor to select a diverse set of configurations from the initial data set, second the force field is refitted with singular value decomposition and an atomic broadening of 0.5 Å and 8 radial basis functions. The force weight is increased to 100 in the fitting procedure, since we are only interested in accurate vibrational properties, whereas the internal energies for the construction of the cluster expansion are taken directly from DFT.