The corresponding force field Hessians were computed by locally minimizing the QM geometries with the force field, followed by evaluating forces with numerical displacements (0.001 Å). Normal mode analysis was carried out and the QM and FF frequencies were sorted from lowest to highest to yield the sorted sequences , and , , respectively. The objective function contribution for each set of normal modes was computed as the sum of squared differences of corresponding frequencies, scaled by a factor of vib = 200cm -1 , as: