Although the existence of Cα−H···OC hydrogen bonds in protein structures recently has been established, little is known about their strength and, therefore, the relative importance of these interactions. We have discovered that similar interactions occur in N,N-dimethylformamide dimers. High level ab initio calculations (MP2/aug-cc-pTZV) yield electronic association energies (D e) and association enthalpies (ΔH 298) for four dimer geometries. These data provide a lower limit of D e = −2.1 kcal mol-1 for the Cα−H···OC hydrogen bond. A linear correlation between C−H···O bond energies and gas-phase proton affinities is reported. The gas-phase anion proton affinity of a peptide Cα−H hydrogen was calculated (355 kcal mol-1) and used to estimate values of D e = −4.0 ± 0.5 kcal mol-1 and ΔH 298 = −3.0 ± 0.5 kcal mol-1 for the Cα−H···OC hydrogen bond. The magnitude of this interaction, roughly one-half the strength of the N−H···OC hydrogen bond, suggests that Cα−H···OC hydrogen bonding interactions represent a hitherto unrecognized, significant contribution in the determination of protein conformation.

Specialized computational chemistry packages have permanently reshaped the landscape of chemical and materials science by providing tools to support and guide experimental efforts and for the prediction of atomistic and electronic properties. In this regard, electronic structure packages have played a special role by using first-principle-driven methodologies to model complex chemical and materials processes. Over the past few decades, the rapid development of computing technologies and the tremendous increase in computational power have offered a unique chance to study complex transformations using sophisticated and predictive many-body techniques that describe correlated behavior of electrons in molecular and condensed phase systems at different levels of theory. In enabling these simulations, novel parallel algorithms have been able to take advantage of computational resources to address the polynomial scaling of electronic structure methods. In this paper, we briefly review the NWChem computational chemistry suite, including its history, design principles, parallel tools, current capabilities, outreach, and outlook.

The structures of the dimers of formamide and N-methylacetamide have been calculated at the ab initio electronic structure theory level, second-order Møller-Plesset perturbation theory (MP2) with augmented correlation consistent basis sets. Five unique structures were optimized for the formamide dimers at the MP2/ aug-cc-pVDZ and MP2/aug-cc-pVTZ levels. At the optimized geometries obtained with the aug-cc-pVTZ basis set, MP2 energies were evaluated with the aug-cc-pVQZ basis set, allowing an extrapolation of the energies to the complete basis set limit. Four structures were found for the N-methylacetamide dimer at the MP2/aug-cc-pVDZ level, and single-point energies were calculated at the MP2/aug-cc-pVTZ level. In both systems, the basis set superposition error was estimated with the counterpoise method. The strength of the NsH‚‚‚OdC bond has a mean value of 7.1 kcal/mol in the formamide dimers and a mean value of 8.6 kcal/mol in the N-methylacetamide dimers. The difference in hydrogen bond strengths is attributed to differences in basicity at the carbonyl oxygen receptor site. In several dimers CsH‚‚‚OdC hydrogen bonds play an important role in stabilizing these intermolecular complexes, increasing the interaction energy by 1.1-2.6 kcal/mol per interaction.