Electron densities of hydrogen atoms

Structures of actual enzyme-substrate complexes are generally difficult to determine, because the reaction occurs too quickly, but techniques now available occasionally enable study of these complexes [53]. Protein X-ray crystallography has several limitations, for example, it often gives little or no information about the positions of protons (because of the low electron density of hydrogen atoms) in a particular protein. This can cause prob-... 

In order to ensure reasonably well-converged results for the optical rotation, basis sets of polarized valence double-zeta quality are required, and sets such as the aug-cc-pVDZ of Woon and Dunning (1994), or the polarized triple-zeta basis of Sadlej (1988) have been shown to perform well for calculations of optical rotation. Most importantly, diffuse p functions in the outer regions of the electron density, which in most cases is described by the electron density of hydrogen atoms, are required to ensure qualitatively correct results (Zuber and Hug 2004). 

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