Search

We present calculations of the optical response of the DNA bases and base pairs both in their normal and tautomeric forms in the gas phase, using time-dependent density functional theory (TDDFT). These calculations are performed in real time within the adiabatic approximation with a basis of local orbitals. Our results for the individual bases are in good agreement with experiment and computationally more demanding calculations of chemical accuracy. The optical response of base pairs indicates that the differences between normal and tautomeric forms in certain cases are significant enough to provide a means of identification.

High Pressure Air Injection (HPAI) is a potentially attractive enhanced oil recovery method for deep, high-pressure light oil reservoirs after waterflooding. The advantage of air over other injectants, like hydrocarbon gas, carbon dioxide, nitrogen, or flue gas, is its availability at any location. HPAI has been successfully applied in the Williston Basin for more than twenty years and is currently being considered by many operators for application in their assets.Evaluation of the applicability of HPAI requires conducting laboratory experiments under reservoir temperature and pressure conditions to confirm crude auto-ignition and to assess the burn characteristics of the crude/reservoir rock system. The ensuing estimation of the potential incremental recovery from the application of HPAI in the reservoir under consideration requires fit-for-purpose numerical modeling. Typically, the flue gas generated in-situ by combustion leads to in an immiscible gas drive, where the stripping of volatile components is a key recovery mechanism. HPAI has therefore, in some instances, been modeled as an isothermal flue gas drive, employing an Equation of State (EOS) methodology. This approach, however, neglects combustion and its effects on both displacement and sweep. Furthermore, the EOS approach cannot predict if, and when, oxygen breakthrough at producers occurs. Combustion can be included in a limited fashion in simulations at the expense of extra computational time and complexity. In the available literature, combustion is taken generally into account under quite simplified conditions.This paper addresses the role that combustion plays on the incremental recovery of HPAI. Numerical simulations were conducted in a 3D model with real geological features. In order to capture more realistically the physics of the combustion front, a reservoir simulator with dynamic gridding capabilities was used. Kinetic parameters were based on the combustion tube laboratory experiments. The impact of combustion on residual oil, sweep efficiency and predicted project lifetime is presented by comparing isothermal EOS-simulations and multi-component combustion runs.

Melanin is a ubiquitous pigment in living organisms with multiple important functions, yet its structure is not well understood. We propose a structural model for eumelanin protomolecules, consisting of 4 or 5 of the basic molecular units (hydroquinone, indolequinone, and its tautomers), in arrangements that contain an inner porphyrin ring. We use time-dependent density functional theory to calculate the optical absorption spectrum of the structural model, which reproduces convincingly the main features of the experimental spectrum of eumelanin. Our model also reproduces accurately other important properties of eumelanin, including x-ray scattering data, its ability to capture and release metal ions, and the characteristic size of the protomolecules. Phys Rev Lett

We present the calculation of the optical response of hydrogen-passivated germanium and silicon clusters using time-dependent density functional theory in the adiabatic local density approximation. The optical response, optical gaps, and other quantities of interest of Ge clusters are compared with the corresponding results for the Si clusters. Our most notable find is that in the low-energy region there are significant differences. More specifically, the Ge clusters exhibit higher oscillator strength and their energy gaps are smaller compared to Si clusters. The overall shape of the optical response of Ge clusters is very similar to that of Si clusters. We conclude that many similarities and differences of these clusters originate from the optical response of the corresponding bulk materials.

We investigate the effect of the coupling of the forward- and backward-going electron-hole pairs on the static local-field factor of jellium with and without a gap. We calculate the static local-field factor for the two types of jellium as a function of $\mathbf{q}$ using the Bethe-Salpeter equation. We assume that a particle and a hole interact via a statically screened Coulomb interaction. The polarization diagrams that include this interaction are summed to infinite order, leading to a matrix equation for the inverse of the polarization propagator. Employing a string of manipulations we convert the matrices in convenient forms, and we then invert the resulting matrices iteratively. This allows us to use matrices of very large size, something that would have not been feasible with straightforward inversion, and therefore achieve a very high level of convergence of results with respect to sampling of electron states in the Fermi sea with relative computational ease. For the calculation of the static local-field factor, we find that the coupling of both kinds of pairs gives qualitatively different results compared to the case when there is no coupling, and that the coupling of both kinds of electron-hole pairs is necessary for obtaining the most accurate results. We compare our results with recent calculations and point out the similarities and differences.

We report on a general method for the calculation of the frequency-dependent optical response of clusters based upon time-dependent density functional theory (TDDFT). The implementation is done using explicit propagation in the time domain and a self-consistent program that uses a linear combination of atomic orbitals (LCAO). Our actual calculations employ the SIESTA program, which is designed to be fast and accurate for large clusters. We use the adiabatic local density approximation to account for exchange and correlation effects. Results are presented for the imaginary part of the linear polarizability, $\mathrm{Im}\ensuremath{\alpha}(\ensuremath{\omega}),$ and the dipole strength function, $S(\ensuremath{\omega}),$ of ${\mathrm{C}}_{60}$ and ${\mathrm{Na}}_{8},$ and compared to previous calculations and to experiment. We also develop a method for the calculation of the integrated frequency-dependent second-order nonlinear polarizability for the case of a step function electric field, ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\gamma}}}_{\mathrm{step}}(\ensuremath{\omega}),$ and present results for ${\mathrm{C}}_{60}.$