The performance of basis sets made of numerical atomic orbitals is explored in density-functional calculations of solids and molecules. With the aim of optimizing basis quality while maintaining strict localization of the orbitals, as needed for linear-scaling calculations, several schemes have been tried. The best performance is obtained for the basis sets generated according to a new scheme presented here, a flexibilization of previous proposals. The basis sets are tested versus converged plane-wave calculations on a significant variety of systems, including covalent, ionic and metallic. Satisfactory convergence (deviations significantly smaller than the accuracy of the underlying theory) is obtained for reasonably small basis sizes, with a clear improvement over previous schemes. The transferability of the obtained basis sets is tested in several cases and it is found to be satisfactory as well.

We present a very efficient and accurate method to simulate scanning tunneling microscopy images and spectra from first-principles density functional calculations. The wave-functions of the tip and sample are calculated separately on the same footing, and propagated far from the surface using the vacuum Green's function. This allows to express the Bardeen matrix elements in terms of convolutions, and to obtain the tunneling current at all tip positions and bias voltages in a single calculation. The efficiency of the method opens the door to real time determination of both tip and surface composition and structure, by comparing experiments to simulated images for a variety of precomputed tips. Comparison with the experimental topography and spectra of the Si(111)-(7×7) surface show a much better agreement with Si than with W tips, implying that the metallic tip is terminated by silicon.PACS numbers: 68.37. Ef, 07.79.Cz, 68.35.Bs The development of scanning tunneling microscopy [1] (STM) and spectroscopy (STS) has provided an unprecedented knowledge about a rich variety of surface science aspects [2,3]. STM and STS convey information about the local geometric and electronic structure of metallic and semiconducting surfaces, which has extensively served to unravel their atomic arrangements and reconstructions, but also to analyze surface defects, study adsorbate-covered solid samples or monitor dynamic surface processes like oxidation or diffusion, to give only some examples. The feasibility of atom by atom chemical analysis was dreamed of since an early stage, due to the unique combination of spatial and energetic resolutions. Currently, STM achieves atomic resolution routinely, while discrimination between atomic species has been reported in particular cases. However, a detailed structural analysis, directly from the experiments, is generally far from a minor task, because structural and electronic properties intermixed in the STM images. An even more fundamental difficulty is the lack of control and knowledge on the composition and structure of the tip. This is particularly crucial in the case of STS, where tip states can entirely modify the spectra. As a result of these uncertainties, a careful comparison with theoretical simulations is generally needed to interpret safely the experimental information. To such an end, much progress has been done towards the theory of STM [4] since the pioneering use of perturbation theory by Bardeen [5]. Tersoff and Hamann (TH) [6], made the additional assumption that the tunnel current is dominated by a single s-state of the tip, what leads to a simple expression involving only the local density of states (DOS) of the surface. When experiments are highly reproducible, regardless of the tip used, this can be sufficient. However, approaches beyond TH [2,7,8], which include the electronic structure of the tip, have been necessary to explain many observations, like bias-dependent images or negative differential resistances. On the other hand, non-perturbative approaches, which ...

Based on Bardeen's perturbative approach to tunneling, we have found an expression for the current between tip and sample, which can be efficiently coded in order to perform fast ab initio simulations of STM images. Under the observation that the potential between the electrodes should be nearly flat at typical tunnel gaps, we have addressed the difficulty in the computation of the tunneling matrix elements by considering a vacuum region of constant potential delimited by two surfaces (each of them close to tip and sample respectively), then propagating tip and sample wave functions by means of the vacuum Green's function, to finally obtain a closed form in terms of convolutions. The current is then computed for every tip-sample relative position and for every bias voltage in one shot. The electronic structure of tip and sample is calculated at the same footing, within density functional theory, and independently. This allows us to carry out multiple simulations for a given surface with a database of different tips. We have applied this method to the Si(111)-(7 × 7) and Ge(111)-c(2 × 8) surfaces. Topographies and spectroscopic data, showing a very good agreement with experiments, are presented.

Post-harvest application of acetaldehyde vapors to blueberries, tomatoes, and pears led to the enhancement of the fruit sensory quality including an increase in the sugar content, sugar-acid ratio, and flavor changes judged to be acceptable by test panels. Applied ethanol vapors led to similar but limited enhancement of fruit sensory quality. Comparative applications of ethylene were more effective in stimulating changes in the background color, including an increase in the total carotenoid content in tomato and .anthocyanines in blueberry. Ethylene, however, had little or no effect, and occasionally led to deleterious changes in the fruit sensory quality. Acetaldehyde and related volatiles may be important in the development of fruit sensory quality, as occurring normally during ripening, or as a post-harvest application for the improvement of fruit sensory quality.

Scanning tunneling microscopy ͑STM͒ experiments and density-functional theory ͑DFT͒ calculations are combined to unravel the complex shifts and splittings of molecular orbitals ͑MOs͒ for the prototype system of a single -conjugated molecule bonded to a semiconductor surface. Intramolecular resolution in STM images of 3,4,9,10-perylene tetracarboxylic dianhydride ͑PTCDA͒ on Si͑111͒-͑7 ϫ 7͒ cannot be understood as resulting from a simple rigid shift of the MOs of the free molecule. DFT calculations and simulations of STM images with realistic tips show large splittings of the original MOs that contribute in a complex way to the tunnel current and are understood under symmetry and charge-transfer arguments. The system is characterized by a strong, partially ionic covalent bonding involving the carboxyl groups of the PTCDA and the Si dangling bonds.