Some difficulties in computing BSSE-corrected potential surfaces of chemical reactions

We performed a theoretical and numerical analysis of the different 'counter poise correction' (CP) schemes potentially applicable to correct for the ba sis set superposition error (BSSE) in the neighborhood of transition struct ures of chemical reactions. The analysis proved that neither of them is sat isfactory: all CP versions result in either discontinuous potential surface s or yield different energies for the same species in different reactions. Standard CP correction is unavoidable and satisfactory when loosely bound p re- or post-reaction complexes are studied. For transition structures, howe ver, doing no correction is better than any available CP method. (C) 1998 E lsevier Science B.V. All rights reserved.