DANGERS OF COUNTERPOISE CORRECTED HYPERSURFACES - ADVANTAGES OF BASIS-SET SUPERPOSITION IMPROVEMENT

The convergence properties of counterpoise corrected hypersurfaces tow ards the limit of infinite basis set size within a specific method are discussed. In the case of concerted hydrogen exchange of the cyclic h ydrogen fluoride trimer counterpoise correction is shown to be highly counterproductive for the convergence speed towards the basis set limi t. A best estimate for the MP2 basis set limit of - 15.2 kcal/mol for the stabilization energy and of 18.7 kcal/mol for the concerted hydrog en exchange barrier of the cyclic hydrogen fluoride trimer is deduced by calculations using basis sets up to aug-cc-pV5Z. Both results are v ery well predicted in the uncorrected case already with the smaller ba sis sets in the MP2/aug-cc-pVxZ series, whereas counterpoise correctio n completely fails to improve predictions mainly due to a wrong correc tion of the dynamic electron correlation contribution. Only with very large basis sets it is possible to obtain counterpoise corrected resul ts of comparable quality. (C) 1998 American Institute of Physics.