AB-INITIO AND REGULARIZED FORCE-FIELDS OF HALOETHANES - CH3CH2CL, CH3CHCL2, CH3CF2CL, AND CH3CFCL2

We present a systematic application of Tikhonov's regularization metho d to joint treatment of ab initio and experimental vibrational data fo r a series of mono-, di-, and tri-substituted haloethanes, based on un iform MP2/6-31G level of theory. This leads to MP2/6-31G*-''regulariz ed'' force field parameters that exactly reproduce experimental freque ncies and isotopomer shifts (within prescribed error levels) and are ' 'closest'' (in Euclidean norm) to the ab initio force field. We correc t a number of previous experimental spectral assignments and investiga te the limits of transferability (e.g., of methyl-group force field pa rameters from one species to another) and other common simplifying app roximations (e.g., MVFF modified valence force field treatment). Our r esults demonstrate how one can systematically combine experimental and ab initio information to create a data base of freon-type force field parameters having greater accuracy and reliability than could be obta ined from theory or experiment alone. (C) 1998 American Institute of P hysics. [S0021-9606(97)01018-0].