On the convergence of the (ΔECCSD(T)-ΔEMP2) term for complexes with multiple H-bonds

The MP2 and CCSD(T) interaction energies of the following cyclic complexes were studied: formic acid dimer (FO-FO), formamide dimer (FA-FA), formamidine dimer (FI-FI), formamide(...)formamidine complex (FA-FI) and the formamidinoaldehyde(...)amidinoformamide (FL-AF) complex. Various AO basis sets up to the cc-pVTZ were investigated. It was shown that the (DeltaE(CCSD(T)) - DeltaE(MP2)) correction term evaluated with the 6-31G* basis set is overestimated while rather accurate values were obtained with 6-31G*(0.25) and cc-pVDZ (0.25, 0.15) basis sets. Because the latter one performs well also for stacked complexes it can be recommended for evaluation of the correction term of extended complexes possessing both H-bonded and stacked structures. (C) 2002 Published by Elsevier Science B.V.