Exchange energy calculation of He-He with supermolecular one-matrix

Exch~nge energy of the He-He system is calculated using the one-density matrix which has been modified according to the supermolecular density formula quoted by Kolos. The exchange energy integrals are computed analytically and by the Monte Carlo method. The results obtained from both ways compared favourably,with the results obtained from the SCF program HOND

Multiple scattering calculations of large inorganic systems

Self-consistent field multiple scattering (SCF-MS) calculations have been performed on the aluminium trihalides, AlX$sb3$ (X = F, Cl, Br, I) and their dimers, Al$sb2$X$sb6$ and on the large organometallic compounds diphenyltin(IV) dichloride and diphenyllead(IV) dichloride monomers and dimers. All these molecules have nuclear quadrupole resonance (n.q.r.) active nuclei and strong or weak intermolecular halogen interaction. In addition, diphenyltin(IV) dichloride has a Mossbauer active $sp{119}$Sn nuclei to study the isomer shift and quadrupole splitting. Calculations used the Vosko-Wilk-Nusair(VWN) exchange-correlation potential in addition to Slater's X$alpha$ exchange potential and also a quasi-relativistic wavefunction for the heavy nuclei in these molecules. The VWN and X$alpha$ n.q.r. results are compared for the aluminium trihalides. The non-relativistic and relativistic calculations which are very important for the hyperfine interactions are compared for diphenyltin(IV) dichloride and the calculated relativistic Mossbauer isomer shift and quadrupole splitting agree well with experiment. The electric field gradients, which are very sensitive to the charge distribution around the nuclei in the molecules, gave very important information, using the n.q.r. and Mossbauer results, about the intermolecular halogen interactions in these molecules, supporting the conclusions arrived at using the one-electron energies and the HOMO-LUMO gap, and from the wavefunction contours and density maps. The relative Lewis acidity order and ionization potentials are also studied in the aluminium trihalides. Qualitative interpretation and agreement with experiment were found whenever experiment was available. Electronic structure and n.q.r. comparison of the diphenyltin(IV) dichloride and diphenyllead(IV) dichloride monomers and dimers lead to the conclusion that the diphenyltin(IV) dichloride is a monomer and the diphenyllead(IV) dichloride is a polymer