Towards computing the properties of large molecules from atoms-in-molecules (AIM) densities

Abstract

This thesis is divided into two main parts. In the first part, we proposed new three partition weights, namely; Fermi-Dirac, triangle, and Awad weights. In this part, we aim to develop new weights in which the core region close to a nucleus of an atom is assigned to that atom and not to the core or the bond of other atoms. We visually illustrate the molecular radial electron density (RDEN) and bond electron density (BDEN) using the proposed weights. We compared the molecular properties including the total number of electrons Nₑ, the electron-nuclear potential energy Vne, and Coulomb potential energy Vₑₑ which are calculated numerically using the Awad weight with those calculated by Hartree-Fock (HF) wavefunction (the exact values). Also, the computed results using the Awad weight were compared to the computed results obtained using Becke weight. Our findings show that the Awad weight gives better bonding-region representations for both RDEN and BDEN than those obtained using Becke weight. Also, the Awad weight gives better results in computing the molecular properties than those using the Becke weight. In the second part, we proposed a new approach for calculating the total energy of molecules called atoms in molecules density (AIMD). In this part, we present the first version of AIMD. In AIMD, the molecular properties of the target molecules were computed in three steps: (i) generating small molecules (fragments) from molecules of interest, (ii) storing some properties of these fragments in a database, (iii) using the data stored in the database and by using the direct method to compute the molecular properties of the target molecule. The electronic molecular energy (Eᴱᴸᴱ) is obtained using AIMD as follows, Eᴱᴸᴱ = Tsum+Vne[ρ]+J[ρ]