Quantum and Classical Studies of Calcium and Zinc Clusters and of Pyrrole Oligomers




Dai, Yafei

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An all electron hybrid density functional approach (DFT) with very large basis sets was used for studying Ca(2) through Ca(19) and Zn(3) through Zn(11) neutral clusters and their cluster anions. Energetics, structure and vibrational analysis of all these neutral clusters and cluster anions are reported. The calculated electron affinities (EA) are in excellent agreement with experiment. Additionally, the electron detachment binding energies (BE) up to Ca(6^-) and Zn(6^-) were identified by analyzing the ground and excited states of the cluster anions and of their corresponding neutral clusters. The theoretical BE is in very good agreement with experiment for both calcium and zinc cluster anions. Polypyrrole (PPy) is a conjugated polymer prototype of conducting polymers. The energetically preferred spatial conformation and charge distribution of n-Py oligomers (n = 1 - 24) in both the reduced and oxidized phases are obtained and analyzed in this work within the hybrid density functional theory. Binding energies, HOMO-LUMO gap energies, radius of gyration, end-to-end distance and vibrational frequencies are reported as a function of oligomer length. The band structure of infinite PPy gives a band gap energy in excellent agreement with experiment for reduced PPy. Evolution of the band gap and the charge-localized states as a function of PPy oxidation level is reported. Based on the DFT results of n-Py oligomers, a classical potential model to treat dense systems of PPy is developed within a modified rigid-ion polarizable force field. This model potential is then used in the Adaptive Tempering Monte Carlo and the Metropolis Monte Carlo simulations of a 192 4-Py system and 64 12-Py system at different densities. The energy, end to end distance, radius of gyration and order parameter as a function of density are inspected. It is shown that these systems have the structural characteristics of liquids. However, the calculations show that as the density is increased, the system develops regions of stacked chains.



Calcium Cluster, Pyrrole Oligomer, Monte Carlo, Zinc Cluster, Density Functional Theory, Electron Detachment Spectra