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 (ΔECCSD(T)−ΔEMP2) 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.