DERIVATION OF CLASS-II FORCE-FIELDS .3. CHARACTERIZATION OF A QUANTUMFORCE-FIELD FOR ALKANES

Recently, a quantum mechanical Class II force field (QMFF) was derived from a fit of HF/6-31G ab initio energy and energy derivative data f or alkanes, and a comparison of this quantum force field and the ab in itio energy and energy derivatives was presented. In this work, the qu antum force field is further evaluated with regard to its accuracy, an d, more importantly, transferability. A detailed comparison between st ructures, frequencies, and energies calculated from quantum mechanics and from the classical analytical form is given for a set of molecules selected from both those used in the original training set and molecu les selected from outside the training set. None of these properties w ere used directly in the original derivation of the force field. In or der to assess the importance of anharmonic and coupling interactions t hat occur in and contribute to molecular energy surfaces, the results are compared to a diagnoal quadratic force field. It is demonstrated t hat the QMFF functional form is capable of calculating the ab initio b ond lengths, bond angles, torsion angles, and conformational energy di fferences to an rms accuracy of 0.003 angstrom, 0.4-degrees, 1.2-degre es, and 1.0 kcal/mol, respectively. This compares quite well to corres ponding deviations of 0.006 angstrom, 0.8 angstrom, 2.3-degrees, and 3 .3 kcal/mol for a harmonic diagnol force field. Excluding three- or fo ur-membered rings, the QMFF rms frequency deviations were 24 cm-1, whi ch again is much better than the approximately 100 cm-1 deviations for the harmonic diagnoal force field. Larger average rms frequency devia tions of 36 and 71 cm-1 were found with QMFF for molecules with three- and four-membered rings. An in-depth analysis of C-H and C-C bond len gth, H-C-H, H-C-C, and C-C-C bond angle, and C-C-C-C torsion angle dev iations is also presented, along with a similar characterization of fr equency deviations in C-H stretching, C-C stretching, C-C-C bending, a nd torsion modes. It is concluded from these results that the use of q uantum energy surfaces allows us to derive a (Class II) functional for m which is not only more accurate, but also more transferable than pre vious generation force fields.