Four dimer scan cases used in the training set to optimize HB parameters. In Scan 1, the water molecule on the right is scanned toward the water molecule on the left in the xy plane, in Scan 2, the donor molecule is rotating around the z axis with the O_i as the center of rotation, in Scan 3, the donor molecule is scanned toward the acceptor with O_iH_jO_k = 180.0° and in the xy plane, and in Scan 4, the donor water molecule is rotating in the xy plane around O_k as the center of rotation. In all cases, the bonds and angles are kept fixed during the scans.