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Using a valence force field, we predict the symmetric lattice distortions around isovalent impurities in 64 semiconductor-impurity systems. For the five systems for which extended x-ray absorption fine-structure (EXAFS) data are available, the results are in excellent agreement with experiment. Our theory also explains quantitatively, without adjustable parameters, the observed bond-length variations in solid solutions A l "B"Cof semiconductor alloys, as well as their excess enthalpies of mixing. cQ P~B (x) 6= 1-=-==-.'Ro Bc Q. Ggi .~' '-'expt ()c) o' Rec CC Rexp~' (X)~Õ CC D~c 0 CQ R' AC AC Composition x of Ag &&BxC AC BC FKJ. 1. Schematic variation of the bond lengths Rgg(x) and R~c(x) in an A l "B"Calloy with composition, depicting the prediction of VCA, Pauling's model [R~~(x) and Ra~{x)] and experiment [Rzc '(x), and Rzc '(x)]. Here, Rzz and Rz& denote the bond lengths in the pure, end-point compounds.

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We present pseudopotential local-density calculations of the electronic and structural properties of solid Ceo (fullerite). The calculated molecular bond lengths, lattice constant, bulk modulus, enthalpy of formation, and the equation of state for compression are in good agreement with experiment. The shape of the theoretical density of states is in excellent agreement with the experimental photoemission and inverse photoemission spectra. We also present the calculated band structure for the states near the fundamental gap. We have made a thorough analysis of the electronic states of C6p and found that they can be conveniently classified according to their angular character, and use it to identify the origin of the peaks in the electronic density of states