Quantum Monte Carlo Calculations of the Electronic excitations of GE atoms, molecules and Nanoclusters Using Core-polarization Potentials
Vincent, Jordan Eric
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Quantum Monte Carlo Calculations of the Electronic excitations of GE atoms, molecules and Nanoclusters Using Core-polarization Potentials
In this thesis many-body Quantum Monte Carlo (QMC) calculations are presented for the ground state and excitation energies of Ge atoms, molecules, and clusters as large as Ge29H36 (≈ 1.2 nm diameter). The hydrogen terminated GenHm nanoclusters are of particular interest since they have optical properties that are different from small molecules and bulk Ge, and are viewed as an ideal model for quantum confined semiconductor systems. The present QMC results are compared with previous QMC calculations for the corresponding Si molecules and clusters. In addition, the QMC gaps for GenHm are found to be higher than the gaps reported in recent time-dependent density functional studies, by amounts similar to that previously found for Si systems. For materials containing Ge it is necessary to deal with the issue of correlation between the core and valence electrons. The accuracy of QMC for heavy atoms such as Ge (Z=32) is limited by the fact that the core-valence interactions cannot be treated at the same manybody level as the valence-valence interactions. Typically the core-valence interactions are treated at a single-body level via a pseudopotential, but such methods are unsatisfactory for Ge which has a shallow, easily polarizable 3d core. Previous work has proposed using a Hartree-Fock pseudopotential (relativistic) plus a core-polarization potential (CPP) to account for core-valance correlation at a many-body level. A major part of this work involves quantifying the effect of core-valence correlation via the CPP. The CPP is found to be important for accurate calculations of the total energy, and for excitations of atoms and small molecules. However, there are only small changes in the lowest optical excitations of larger clusters, which can be understood in terms of the nature
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