  In this thesis a new model for calculating the total energy of atomic and
solid systems is presented. The model is used to study the properties of the
Si(100) surface and dislocation dynamics in the diamond structure. For the
Si(100) surface the main result is the finding of an incomplete melted state at
$1550^0 K$. The studied dislocation is the $90^0$ partial and it is found that
the barriers for kink formation and migration are in good accordance with
experimental observations. The general idea for calculating the total energy
for a given system is to relate a reference system to each atom, an effective
medium for which the total energy is known and then only calculate the energy
difference between the two systems. Using transferable potentials and densities
a pair potential is constructed which describe the electro-static and
exchange-correlation part of the energy difference. The remaining part, the
one-electron difference, is calculated with an LMTO tight-binding model with
the potential parameters determined from the effective potential of the
reference system.
