JOINT TREATMENT OF AB-INITIO AND EXPERIMENTAL-DATA IN MOLECULAR-FORCEFIELD CALCULATIONS WITH TIKHONOVS METHOD OF REGULARIZATION

We describe a novel method for employing calculated ab initio potentia l data together with Tikhonov's variational procedure to extract funda mental molecular force field parameters from experimental spectral dat a, the formal ''inverse problem'' of vibrational spectroscopy. In this approach, the ab initio quantities serve to ''regularize'' the initia lly ill-posed problem (in the sense of Tikhonov), leading to variation ally stable and unique force field parameters that optimally mimic ove rall patterns of the (approximate) ab initio quantities, but exactly r eproduce the available experimental data within specified experimental precision. In this manner, ab initio and experimental data can be joi ntly combined to produce more stable and reliable force fields (improv able to any degree through higher level ab initio treatment, additiona l experimental data, etc.) than could be attained by theoretical or ex perimental methods alone. The proposed procedure allows use of any sys tem of generalized coordinates, including redundant systems of interna l or symmetry coordinates, simplifying the transfer and comparison of force constants in series of related molecules. The procedure is illus trated with numerical application to CHF2Cl and its isotopomers at MP2 /3-21G, MP2/6-31G* levels of theory, demonstrating the stability and consistency of force obtained from different levels of theoretical inp ut.