Real space multiple scattering calculations of the electronic density of states and x-ray spectra in solids typically scale as the cube of the system and basis set size, and hence are highly demanding computationally. For example, x-ray absorption near edge structure (XANES) calculations in solids typically require clusters of order NR atoms and s, p, and d states for convergence, with NR between about 10 2-10 3 ; for this case about 10 2 inversions of 9NR × 9NR matrices are needed, one for each energy point. We discuss here two ways to speed up these calculations: 1) message passing interface (MPI) parallel processing, and 2) fast, Lanczos multiple scattering algorithms. Together these algorithms can reduce computation times typically by two orders of magnitude. These are both implemented in a generalization of the ab initio self-consistent FEFF8 code, which thus makes XANES calculations in complex systems with of order 10 3 atoms practical. The Lanczos algorithm also yields a natural crossover between full and finite-order multiple scattering with increasing energy, thus differentiating the extended and near-edge regimes.

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Extended x-ray-absorption fine structure ͑EXAFS͒, Fourier-transform infrared absorption ͑FTIR͒, and elemental analysis were used on a variety of CdSe nanocrystallites ͑NC's͒ to study surface structure. All CdSe NC's were grown by standard inverse micelle techniques. Two sets of NC's samples were made. One set was made so that only the size of the NC's was varied, while the surface treatment was kept the same. The other set was made so that only the surface treatment was varied, while the size distribution was kept the same. For the EXAFS experiments, reference compounds similar in structure were measured. FTIR found surface Cd atoms to be passivated by pyridines and water groups. Fourier-filtered first-shell Cd EXAFS also supports the existence of water groups attached to the surface Cd atoms. The lack of any SeO and Si(CH 3 ) 3 in the FTIR signal indicates that most surface Se atoms have unterminated bonds. Fourier-filtered first-shell Se EXAFS spectra indicate that Se has only Cd as its first-nearest neighbor, and that the coordination number is reduced from the bulk value, suggesting surface Se atoms are unpassivated. Our data support the existence of surface Se lone-pair orbitals that can trap an optically excited hole. ͓S0163-1829͑97͒00908-9͔

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