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The story is old but needs to be told. There was a time when quantum confinement (QC) was an embarrassment. A calculation which yielded a larger than bulk band gap (say 2 eV instead of 1.17 eV for Si) invited incredulous queries and even derision.
Shallow impurities in semiconductors had a well-defined and accepted theory since the late 1950s. Kohn and Luttinger had proposed the effective mass theory (EMT) and one knew how to calculate the donor and acceptor ionization energies with confidence. The problem was with defects whose energy levels lay deep inside the the fundamental gap. Deep defect energies could not be predicted on the basis of EMT. These defects ruptured and rearranged bonds, diffused in anomalous ways and in general behaved in erratic and roughish ways.
Since deep levels were found well inside the fundamental gap their wavefunctions were expected to be localized. One such defect investigated by the authors was hydrogen which was surrounded by a cluster of ``host'' silicon atoms a little over a nanometer in size. A sophisticated non-orthogonal many neighbours tight-binding approach was employed. The gap, the electronic level(s) of hydrogen and the vacancy-hydrogen complex were investigated using this (inadvertent) ``nanostructural'' silicon. The authors went further and calculated the vibrational frequencies and the diffusion barriers of hydrogen.
The use of nanostructural silicon as a host for deep defects was termed the ``defect'' molecule approach. The authors were not the only ones employing such a approach. But we single out this work for a number of reasons. These calculations were carried out before: (i) hydrogen became the ``popular'' passivating defect, a subject of exclusive conferences and volumes; (ii) the advent of amorphous hydrogenated silicon; (iii) the immense popularity of nanostructural systems.
The techniques employed in those ``prehistoric'' days have been rediscovered in the 1990s'. We urge you to take another look at these works: several nuggests lie buried in those publications. The one limitation of those days was the inability of the computer systems to handle clusters larger than 100 atoms except under special circumstances. Like we said, the story is old but needs to be told. No point in reinventing the wheel.