Physics

We investigate the nuclear isotropic shielding constants and spin-spin coupling for oxygen and carbons atoms of isomers of tartaric acid in gas phase and water solutions by Monte Carlo simulation and quantum mechanics calculations using the GIAO-B3LYP approach. Solute polarization effects are included iteratively and play an important role in the quantitative determination of shielding constants. Our MP2/aug-cc-pVTZ results show substantial increases of the dipole moment in solution as compared with the gas phase results (61-221%). The solvent effects on the {\sigma}(13C) [J(C-C)] values are in general small. More appreciable solvent effects can be seen on the {\sigma}(17O) and J(C-O).

In this study, a fully self-consistent method was developed to obtain the wave functions of the positron and electrons in molecules simultaneously. The wave function of a positron at room temperature , with a characteristic energy of approximately 0.04eV , was used to analyse the experimental results of its annihilation in helium, neon, hydrogen, and methane molecules. The interactions between the positron and molecule provide a significant correction in the gamma-ray spectra of the annihilating electron--positron pairs. It was also observed that high-order correlations offered almost no correction in the spectra, as the interaction between the low-energy positron and electrons cannot drive the electrons into excited electronic states. More accurate studies, which consider the coupling of the positron--electron pair states and vibration states of nuclei, must be undertaken.

The newly developed compound ExBox4+ is explored to check whether it is a proficient hydrogen storage material. Both exoherdal and endohedral hydrogen adsorption on ExBox4+ are studied. Endohedral hydrogen molecules interact strongly than exohedral ones. The hydrogen adsorption energy is as good as the recently studied charged fullerenes. The hydrogen storage capacity appears to be ~4.3 wt%. The endohedral CO sorption is also analysed with the help of DFT. The first principle DFT calculation and MD simulation are performed to investigate the effect of lithium doping on the gas adsorbing capacity and adsorption enthalpy as well as adsorption energy of ExBox4+.The metal atom interaction with ExBox4+is found to be pretty strong, and the interaction energy appears to be higher than the metal cohesive energy.The thermodynamic parameters suggest that metal doping method is spontaneous in nature. The analysis of adsorption energy, thermodynamic properties and MD simulation results suggest that Li doped ExBox4+ can be a promising gas adsorbing material. Lithium doping increases the gas adsorption capacity of ExBox4+. The Li decorated ExBox4+ is found to adsorb twenty four hydrogen molecules with a gravimetric wt% of 6.23. Adsorption of CO on the metal decorated ExBox4+ is also studied. AIM analysis is performed to obtain a general idea of the bonding interaction between Li-H and Li-C.

We study the static linear response in spherical Thomas-Fermi systems deriving a simple diferen- tial equation for general multipolar moments and associated polarizabilities. We test the equation on sodium clusters between 20 and 100 atoms and on fullerenes between C60 and C240 and propose it for general Thomas-Fermi systems. Our simple method provides results which deviates from experimental data with less then 15%.

The photoionization of chiral molecules prepared in a coherent superposition of excited states can give access to the underlying chiral coherent dynamics in a procedure known as photoexcitation induced photoelectron circular dichroism (PXECD). This exclusive dependence on coherence can also be seen in a different part of the angular spectrum, where it is not contingent on the chirality of the molecule, thus allowing extension of PXECD's sensitivity to tracking coherence to non-chiral molecules. Here we present a general theory of PXECD based on angular momentum algebra and derive explicit expressions for all pertinent asymmetry parameters which arise for arbitrary polarisation of pump and probe pulses. The theory is developed in a way that clearly and simply separates chiral and non-chiral contributions to the photoelectron angular distribution, and also demonstrates how PXECD and PECD-type contributions, which may be distinguished by whether pump or ionizing probe enables chiral response, are mixed when arbitrary polarization is used.

With the aim of describing bound and continuum states for diatomic molecules, we develop and implement a spectral method that makes use of Generalized Sturmian Functions (GSF) in prolate spheroidal coordinates. In order to master all computational issues, we apply here the method to one--electron molecular ions and compare it with benchmark data for both ground and excited states. We actually propose two different computational schemes to solve the two coupled differential equations. The first one is an iterative 1d procedure in which one solves alternately the angular and the radial equations, the latter yielding the state energy. The second, named direct 2d method, consists in representing the Hamiltonian matrix in a two--dimensional GSF basis set, and its further diagonalization. Both spectral schemes are timewise computationally efficient since the basis elements are such that no derivatives have to be calculated numerically. Moreover, very accurate results are obtained with minimal basis sets. This is related on one side to the use of the natural coordinate system and, on the other, to the intrinsic good property of all GSF basis elements that are constructed as to obey appropriate physical boundary conditions. The present implementation for bound states paves the way for the study of continuum states involved in ionization of one or two--electron diatomic targets.

Correlation between geometry, electronic structure and magnetism of solids is both intriguing and elusive. This is particularly strongly manifested in small clusters, where a vast number of unusual structures appear. Here, we employ density functional theory in combination with a genetic search algorithm, GGA +U and a hybrid functional to determine the structure of gas phase Fe x O +/0 y clusters. For Fe x O y cation clusters we also calculate the corresponding vibration spectra and compare them with experiments. We successfully identify Fe 3 O + 4 , Fe 4 O + 5 , Fe 4 O + 6 , Fe 5 O + 7 and propose structures for Fe 6 O + 8 . Within the triangular geometric structure of Fe 3 O + 4 a non-collinear, ferrimagnetic and ferromagnetic state are comparable in energy. Fe 4 O + 5 and Fe 4 O + 6 are ferrimagnetic with a residual magnetic moment of 1~\muB{} due to ionization. Fe 5 O + 7 is ferrimagnetic due to the odd number of Fe atoms. We compare the electronic structure with bulk magnetite and find Fe 4 O + 5 , Fe 4 O + 6 , Fe 6 O + 8 to be mixed valence clusters. In contrast, in Fe 3 O + 4 and Fe 5 O + 7 all Fe are found to be trivalent.

Based on a recently introduced metric for measuring distances between configurations, we in- troduce distance-energy (DE) plots to characterize the potential energy surface (PES) of clusters. Producing such plots is computationally feasible on the density functional (DFT) level since it re- quires only a set of a few hundred stable low energy configurations including the global minimum. By comparison with standard criteria based on disconnectivity graphs and on the dynamics of Lennard- Jones clusters we show that the DE plots convey the necessary information about the character of the potential energy surface and allow to distinguish between glassy and non-glassy systems. We then apply this analysis to real systems on the DFT level and show that both glassy and non-glassy clusters can be found in simulations. It however turns out that among our investigated clusters only those can be synthesized experimentally which exhibit a non-glassy landscape.

The variational Monte Carlo method is used to evaluate the ground-state energy of the confined hydrogen molecule, H_2. Accordingly, we considered the case of hydrogen molecule confined by a hard prolate spheroidal cavity when the nuclear positions are clamped at the foci (on-focus case). Also, the case of off-focus nuclei in which the two nuclei are not clamped to the foci is studied. This case provides flexibility for the treatment of the molecular properties by selecting an arbitrary size and shape of the confining spheroidal box. An accurate trial wave function depending on many variational parameters is used for this purpose. The obtained results are in good agreement with the most recent results.

The ground state energy of hydrogen molecular ion H2+ confined by a hard prolate spheroidal cavity is calculated. The case in which the nuclear positions are clamped at the foci is considered. Our calculations are based on using the variational Monte Carlo method with an accurate trial wave function depending on many variational parameters. The calculations were extended also to include the HeH++ molecular ion. The obtained results are in good agreement with the recent results.