Transferability of charge density parameters among related atoms

2 Transferability of Charge Density Parameters Among Related Atoms 

Transferability of atomic densities was tested by Brock et al. (1991), who applied atomic charge density parameters from an accurate low-temperature study of perylene (I) to data collected at five and six different temperatures on naphthalene (II) and anthracene (III), respectively. The molecules are all aromatic hydrocarbons. To reduce the number of variables, all H atoms were assigned 

Parameters not listed are equal to zero. C carbon atom in C=0 group O oxygen atom in 0=0 group N nitrogen atom in peptide bond H generalized hydrogen atom C(sp3)cr carbon atoms bonded to two C, one H, and one N atom, C(sp3)0 bonded to two C and two H atoms C(sp2) phenyl carbon atom. Coordinate systems are defined in 

The deformation of the peptide nitrogen atom is described by a single d33 + ( = x3 - 3xy2) function, the z axis being perpendicular to the C—N—C plane. The deformations of three types of atoms are illustrated in Fig. 12.2. 

As a test, the standard aspherical form factors in their proper orientations were applied in a refinement of both room temperature and low temperature (125 K), accurate, but low-resolution, data ( 0.7 A -1) on the tripeptide pGlu-Phe-D-Pro-0[CN4]-Me (PPP). The improvements on introduction of the aspherical scattering factors are dramatic, the /(-factors being reduced by 30-40% without the introduction of additional variables. Significant adjustments in the anisotropic 

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