On the other hand, CGENFF relies heavily on ab initio calculations. CGENFF I charges can be first assigned by a similarity search through a library of parametrized fragments or can be derived using MP2/6-31G(d) Merz-Kollman charges. With either starting guess, the charges are subsequently optimized by fitting to QM-calculated scaled interaction energies at the HF/6-31G(d) level between the molecule and water in a variety of conformations.