A INTRODUCTION

Local deviations from the overall periodicity of the silicon crystal (i.e. bulk defects and surfaces) forfeit the principal basis for applying conventional band theory in structure calculations. Also, the use of the customary and otherwise very convenient plane wave expansion for one-electron states may prove to be very time consuming, especially for light impurities. Instead, localised states of the defect environment can be thought of as a 'defect molecule' [1], so invoking quantum chemistry, i.e. molecular physics, seems obvious. Of course, the defect molecule is embedded in an otherwise perfect crystalline environment. This can be taken into account in a simple way by adding a sizeable part of the nominally unperturbed portion of the lattice to the defect molecule, to form a so called cluster of atoms. The artificial states introduced into the 'gap' of the cluster by the dangling bonds on its surface should, however, be eliminated. One way to do that is to saturate the dangling bonds by hydrogen atoms: the resulting molecular cluster model (MCM) is very simple and has been widely used [2]. Another way is to choose the cluster in the form of a unit cell and to apply the Born K rm n cyclic boundary conditions, without applying Bloch's theorem [3,4]. With appropriate choice of the unit cell (to represent a special sampling of the primitive Brillouin zone [5]), and proper handling of...