Interatomic spatial correlations

Interatomic spatial correlations are represented by the radial distribution function, D r), which can be obtained by the X-ray and neutron diffraction methods. From the peak position, the peak area, and the peak width, the interatomic distance, r, the number of atoms within the atom pair, n and the mean-square amplitude of the distance of the atom-pair, a, respectively, can be estimated. 

The reduced RDF of a-Si H shown in Fig. 2.11 (Schulke 1981) has sharp structure at small interatomic distances, progressively less well-defined peaks at larger distances, and is featureless beyond about 10 A. This reflects the common property of all covalent amorphous semiconductors, that there is a high degree of short range order at the first and second neighbor distances, but then the spatial correlations decrease rapidly. 

Using the same kind of augmented correlation-consistent basis sets employed in the above coupled cluster methods, but working with the complete active subspace (CASSCF and CASSCF( +1, +2)) approximations to the Cl expansion, Lawson and Harrison101 have investigated the variation with interatomic distance and spatial distribution of the quadrupole moments of P2, S2 and CI2. The a and it contributions to the quadrupole are resolved and the poor results obtained at the SCF level are attributed to the inadequate representation of the it system in the SCF approximation for P2 and S2. 

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