Basis set limit binding energies of hydrogen bonded clusters

The utility of the recently developed extrapolation method to estimate the binding energies of weakly bound clusters at the basis set limit exploiting the similar basis set convergence behaviour of correlation energies of the monomer and cluster in correlated calculations (J. chem. Phys., 116, 5389 (2002)) was tested for small to medium (HF)n and (H2O)n (n = 2–5) clusters using various correlation consistent cc-pVXZ (X= D,T,Q,5) basis sets containing different numbers of diffuse functions and 6–31G type basis sets at the MP2 and CCSD(T) level for which accurate basis set limits are available for comparison. It is shown that the basis set limit binding energies estimated by this extrapolation method with modest size of basis sets (cc-pVDZ/cc-pVTZ or 6–3 1 G(d,p)/6-3 IG(2df,2pd)) are much closer to the exact basis set limits than the estimates by commonly used X −3 extrapolation or counterpoise corrected binding energies, signifying the importance of this extrapolation method for the study of large weakly bound clusters. It is also shown that the inclusion of appropriate diffuse functions in the basis sets can significantly improve the accuracy of the estimated basis set limits by this extrapolation method. For (HF)n clusters the MP2 and CCSD(T) basis set limits estimated by this extrapolation method with aug-cc-pVDZ and aug-cc-pVTZ basis sets are 18.4 (18.5) and 18.9 (18.9) for the dimer, 61.8 (62) and 63.2 (63) for the trimer, 113.5 and 114.7 (116) for the tetramer, and 155.2 and 156.3 (158) for the pentamer, respectively, with the values in parentheses representing the apparent basis set limits, with the numbers in units of kJ mol−1. The corresponding results for (H2O)n clusters are 20.5 (20.5) and 20.6 (20.7) for (H2O)2, and 60.5 (61) and 60.1 (60) for (H2O)3, respectively.