PHD theses : Science

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    Ultrasoft pseudopotentials (US) and projector augmented-Wave data sets (PAW): application to diatomic molecules and solids
    (Aِl neelain universityِ, 2014-06) Alwaleed Ahmed Adllan
    Abstract Several ultrasoft pseudopotentials (US-PPs) and projector augmented-wave (PAW) data sets were generated and tested by calculating the bond lengths, the atomiza- tion energies, the frequencies of the vibrational stretch modes of various diatomic molecules dimers H2, N2, O2, F2, A12, Si2, P2, S2 and O12 and several mono hy- drides, carbides, nitrides and oxides of B, C, N, O, F, Al, S, P, S, Cl, Fe and Ni. These tests were also performed on generated phonon dispersion relations, dielectric constant, lattice constant and the density of states of zinc-blende GaX (X=N, P and AS) solids. The US-PPs and the PAW data sets were constructed with the same recipe using the local density approximation (LDA) or the Perdew, Burke and Ernzerhof (PBE) generalized gradient approximation for the exchange and correlation energies. All calculations were performed using Quantum Espresso software package. The diatomic molecules dimers H2, N2, O2, F2, A12, Si2, P2, S2 and C12 and several mono hydrides, carbides, nitrides and oxides of B, C, N, O, F, Al, S, P, S, Cl, Fe and Ni were studied and found that US-PPs and PAW data sets constructed with the same parameters, provide almost equivalent results for the bond lengths, the vibrational stretch frequencies. For the some molecules, PAW method is found to be superior to the US-PP method for the calculation of the atomization energies. Geometries and vibrational frequencies were compared with some results present in the literature obtained by localized basis sets. It is found that our results are in good agreement with these results, with small discrepancies comparable to those due to the use of different localized basis sets. The solids GaX (X=N,P and As) were also studied and found that US-PPs and PAW data sets constructed with the same parameters, provide almost equivalent results for lattice constant, dielectric constant, density of state and dispersion re- lations. The obtained results and the available literature data (theoretical and experi- mental) were found in good agreement. Moreover, The PBE phonon dispersions relation of GaX compound were found to be much softer than the LDA ones with a substantial underestimation of the optical phonon frequencies. Good agreement between our results of density of states, lattice constants and dielectric constants and experimental data. were also found.