SUMMARY
We present the structural and electronic properties of GaAs under the effect of pressure
using large unit cell method within the framework of intermediate neglect of differential overlap
(LUC-INDO) calculations. It is found that the results are consistent with the available experimental
data and other theoretical results. Energy gap, valence bandwidth, bulk modulus and cohesive energy
increase with increasing pressure, while the conduction bandwidth decreases. All the aforementioned
properties are obtained by selecting empirical parameter sets for LUC-INDO calculation.

Key words: Electronic structure, gallium arsenide, pressure, LUC-INDO, Energy gap, valence
bandwidth, bulk modulus and cohesive energy.

Introduction
Theoretical studies of properties continue to be of great importance. Among
these are the complete (CNDO) and intermediate (INDO) neglect of differential
overlap. The extension of the semi-empirical calculations to solids had begun in
approximately one decade after its application to molecules (9). The extension
made use of the large unit cell (LUC) method to implement periodic boundary of
solid. LUC-INDO calculations have the ability to include large number of atoms
including surfaces, interstitials, or vacancies. Previous LUC-INDO calculations
for semiconductors had focused on diamond and silicon (14, 6). Other calculations
applied the method to F centers (8) titanium oxides (10), etc. These methods
were suggested in the seventies of the last century to overcome computational
problems of ab initio methods for molecules (13). Vast literature accumulated on
the use of semi-empirical methods in solid-state problems (20).