Ab initio restricted Hartree-Fock method coupled with the large unit cell method is used to determine the electronic structure and physical properties of aluminum phosphide (AlP) nanocrystals between 216 and 1000 atoms with sizes ranging up to about 3nm in diameter. Core and surface parts with different sizes are investigated. Investigated properties include total energy, cohesive energy, energy gap, valence band width, ionicity, and degeneracy of energy levels. The oxygenated (001)-(1×1) facet that expands with larger sizes of nanocrystals is investigated to determine the rule of the surface in nanocrystals electronic structure. Results revealed that electronic properties converge to some limit as the size of the large unit cell increases and that the 216 core atoms approaches bulk of Aluminum phosphide material in several properties. Increasing nanocrystals size also resulted in a decrease in lattice constant, increase of core cohesive energy (absolute value), increase of core energy gap, increase of core valence band width and decrease of ionicity. Valence and conduction bands are wider on the surface due to splitting and oxygen atoms. The method also shows fluctuations in the converged energy gap, valence band width and cohesive energy of core part of nanocrystals duo to shape variation.

Self-consistent Hartree-Fock within the framework of large unit cell (LUC) formalism using complete and intermediate neglect of differential overlap (INDO) is used to simulate electronic band structure of covalent crystalline semiconductors. Correlation corrections are added via second-order Moller-Plesset perturbation method. Other corrections such as relativistic spin-orbit effects on the band gap and zero-point energy are also included. Cohesive energy, band gap, valence-band width, and hybridization orbitals are obtained from band calculations. Bulk modulus, charge distribution, and x-ray form factors are also calculated. These calculations are directed toward large scale or defected systems. Results show that the effects of some of these corrections are indispensable to obtain a clear view of semiempirical parameters used to fit band structure of covalent semiconductors. Correlation and INDO corrections removed some electronic cloud from the bonding region to the spherical region around the atom. Correlation corrections with its long time and limited improvements are not advised for large scale calculations. Solid parameters are compared with those of molecules and free atoms to understand the rules of parametrization in solids. Resemblance with size-dependent nanocluster properties is established for present calculations such as the decrease of band gap with increasing size of LUC. The major difference of the present calculations from previous nanocluster calculations is the use of the same element atom of the cluster to cap surface dangling bonds.