Comment on “Strength of the N−H···OC Bonds in Formamide and N-Methylacetamide Dimers”

Abstract

We appreciate the comments of Frey and Leutwyler on our previous work. We agree that we made a mathematical error in the calculation of the BSSE corrected binding energy for the dimerization energy of formamide dimer structure 1 with the aug-cc-pVDZ basis set and appreciate that Frey and Leutwyler have corrected this error. We also agree that our previous calculations were done with all electrons correlated instead of just the valence electrons and that the work presented by Frey and Leutwyler represents a better approximation to the electronic component of the valence electrons to the dissociation energy of the formamide dimer than did our previous work. In order to better understand the role of correlating the 1s electrons with the aug-cc-pVnZ basis sets and extrapolating to the complete basis set (CBS) limit, we have performed two sets of calculations. We first calculated the electronic contribution to the dissociation energy of the water dimer as shown in Table 1. The calculations were done with the geometry optimized at the frozen core MP2/aug-cc-pVTZ level. The calculations show that at the complete basis set (CBS) limit, the results with the core correlated give a well-depth 0.4 kcal/mol deeper than when the core ismore » not included, similar to the difference of 0.5 kcal/mol (out of {approx}15 kcal/mol) that Frey and Leutwyler find for the electronic contribution for the formamide dimer. We also note the hump in the dissociation energy curve for the all electron correlation calculations just as found for the formamide dimer. This hump represents a basis set superposition error (BSSE) which presumably arises because the introduction of more functions with the larger basis sets helps to correlate the core electrons better. We note that the fully correlated and frozen core results for the electronic energy contribution to the dimer dissociation energy when the counter poise correction is included are the same for the H2O dimer just as found by Frey and Leutwyler for the formamide dimer after correcting our mathematical mistake. It is clear that the counter poise correction does allow one to correct for including the core electrons without core-valence correlation functions.« less