IOSR Journal of Applied Physics (IOSR-JAP) e-ISSN: 2278-4861. Volume 4, Issue 5 (Sep. - Oct. 2013), PP 85-95 www.iosrjournals.org www.iosrjournals.org 85 | Page Computations of the Ground State Cohesive Properties Of Alas Crystalline Structure Using Fhi-Aims Code G.S.M. Galadanci 1 , Garba Babaji 2 1,2 (Department of Physics, Bayero University, Kano P.M.B. 3011, Kano Nigeria.) Abstract: A density functional theory codes FHI-aims is used to investigate the material properties of AlAs bulk crystalline structures. The code has several input parameters or variables some of which should be optimized. In the FHI-aims code we study different phases of AlAs crystalline structure and the ground state cohesive properties of the most stable structure of AlAs was computed within GGA and LDA of the density-functional theory. The results of computations shows that the ground state equilibrium properties of and AlAs such as Lattice constants, cohesive energies and Bulk modulus are in agreement with experimentally found values within reasonable percentage errors. Keywords: bcc, bcc, bulk modulus, cohesive energy, crystalline structure, DFT, fcc, ground state, lattice- constant, simple cubic, and total energy. I. Introduction Total Energy calculation and molecular dynamic simulation employing density-functional theory represent a reliable tool in condensed matter physics, material science, and physical chemistry. A large variety of applications such as in molecules, bulk materials and surfaces have proven the power of these methods in analyzing as well as predicting non-equilibrium and equilibrium properties. Density-functional theory (DFT) is one of the most popular and successful quantum mechanical approaches to matter. It is nowadays routinely applied for computations of ground state properties of molecules and solids such as the binding energy of molecules and the band structure of solids in physics. This is a computational material science research work in which the ground state properties of AlAs crystals were investigated using DFT based code FHI-aims as a tool. The equilibrium cohesive properties of AlAs crystal were calculated using FHI-aims code. The program FHI-aims uses DFT as a main production technique to determine electronic and structural properties of molecular or solid condensed matter in its ground state within the local or semi-local approximations[1]. FHI-aims code works on a Linux based operating system in which a FORTRAN-95 or later compiler was installed. A compiled version of lapack libraries and a library providing optimized linier algebra subroutines(BLAS) must also be installed. When the above requirements were made, the FHI-aims executable binary file could then be build. Execution of FHI-aims program requires two input files control.in and geometry.in. The input file contains all the information related to atomic structure and runtime-specific for a given calculation. This research work was initiated due to the importance of AlAs in the modern technology. Some of the noble importance of AlAs are: AlAs is a group III/V binary compound semiconductor. AlAs is a compound of aluminum and arsenic. The crystals of AlAs is technologically important and among the most studied compounds semiconductor material. It is used in the manufacture of devices such as microwave frequency integrated circuits , monolithic microwave integrated circuits , infrared light-emitting diodes , laser diodes , solar cells and optical windows[2. 3](Streetman and Banerjee, 2006; Madelung and Landolt-Börnstein, 1982). Some electronic properties of aluminum arsenide are superior to those of silicon . AlAs has a higher saturated electron velocity and higher electron mobility , allowing its transistors to function at frequencies in excess of 250 GHz. Unlike silicon junctions, AlAs devices are relatively insensitive to heat owing to its wider band-gap. Also, AlAs devices tend to have less noise than silicon devices, especially at high frequencies. This is as a result of higher carrier mobility and lower resistive device parasitics due to radiation. These properties recommend AlAs circuitry in mobile phones , satellite communications, microwave point-to-point links and higher frequency radar systems. It is used in the manufacture of Gunn diodes for generation of microwaves. AlAs have indirect bandgap and so is very poor at emitting light. Nonetheless, recent advances may make silicon and AlAs LEDs and lasers possible. As a wide direct band gap material with resulting resistance to radiation damage, AlAs is an excellent material for space electronics and optical windows in high power applications. Combined with the high dielectric constant, this property makes AlAs very good electrical substrate and unlike Si provides natural isolation between devices and circuits. This has made those materials ideal for microwave and millimeter wave