The lattice vibrational frequencies (phonons) were extracted from the Hessian matrix of the lattice energy within the harmonic approximation. To build the Hessian matrix, we use a finite difference approach where each atom of the crystal is displaced, and density functional theory (DFT) or a molecular mechanics force field is used to compute the force on all the atoms of the crystal upon this displacement. The eigenvalues and eigenvectors of this matrix correspond to the 3 × N vibrational frequencies and vibrational modes of the system, respectively. We use the projection of the eigenvectors on each atom of the crystal to compute the contribution of each atom to the heat capacity, that is used as labels for training the machine learning.