To identify which normal modes to compare from the full and partial QM Hessians, we calculated the overlap, i.e., the dot product, between the C i vectors and the L PH i eigenvectors of the partial QM Hessian. This shows the similarity between normal modes from the full and partial QM Hessians. The most similar normal modes were then used for the second scheme, where the directional deviation between the eigenvectors of the full and partial QM Hessians was calculated. For this, we define the angular deviation AD i of the ith normal mode as