الفهرس 
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Abstract
The thesis comprises three chapters and two appendices. The first
chapter includes an introduction, a literature survey and the aim of this work.
Historical background about the density functional theory (DFT) was given.
The different theories were presented and the accuracy of calculations for
each theory was shown. The accuracy of predicted atomization energies was
within 3-5 kcalJmol, and bond lengths and bond angles were within a few
percent of corresponding experimental values. However, G2, BlL YP, and
even the recently Bx88IBc95 methodologies scaled as ~ :N7, ~, and N5,
respectively, where N is the number of the basis functions. Therefore, there
was a need for a novel method that scales as N3 to apply to large chemical
systems.
In the second chapter, it was my objective to derive the basic working
equations in OFT. The starting point was the definition of the Schrodinger
equation. The concepts of Density functional theory were shown, together
with a comparison of these equations with corresponding ones obtained from
Hartree-Fock theory. The solution of the DFT equations was given.
Molecular orbital theory and basis sets were described. The development of
the new DFT model (HFS-BVWN) that will be used throughout this work
was presented.
The third chapter of the thesis deals with the results and discussion.
The calculated atomic and molecular energies using HFS-BVWN/6-
311+g(3df,p) method are on the average 0.13% higher than the
corresponding correlated HF values. The reported method is thus expected to underestimate molecular exchange-correlation energIes by more than
0.13%. Application of atom equivalent scheme (AES) in connection with
the results of HFS-BVWN/6-311 +g(3df,p) computations gave the corrected
atomic energies for hydrogen through chlorine (H-C!).
The room temperature heats of formation for 118 molecules and
radicals are calculated from the results of HFS-BVWN/6-311 +g(3df,p)
computations and the derived atom equivalents. The average absolute error
in calculated enthalpies of formation is 1.8 kcaUmol. The corresponding
average errors in G2, Bx88/Bc95, B3LYP and BLYP theories are 1.2, 2.0,
2.4 and 3.94 kcallmol respectively. Also the ionization potentials and the
electron affinities of the ions of the atoms of the first two rows of the
periodic table and some selected molecules and radicals are calculated from
the results of HFS-BVWN/6-311+g(3df,p) method. For ionization
potentials (IPs) the average absolute deviation is 0.12 eV, compared with
0.195 eV for BLYP, 0.l4 eV for B3LYP, 0.12 eV for Bx88/Bc95 and 0.05
eV for G2. The calculated electron affinities have an average deviation of
0.13 eV, excluuing hydrogen, compared with 0.08 eV for G2 and 0.14 eV
forBLYP.