Nonlocal van der Waals density functional: The simpler the better

We devise a nonlocal correlation energy functional that describes the entire range of dispersion interactions in a seamless fashion using only the electron density as input. The new functional is considerably simpler than its predecessors of a similar type. The functional has a tractable and robust analytic form that lends itself to efficient self-consistent implementation. When paired with an appropriate exchange functional, our nonlocal correlation model yields accurate interaction energies of weakly-bound complexes, not only near the energy minima but also far from equilibrium. Our model exhibits an outstanding precision at predicting equilibrium intermonomer separations in van der Waals complexes. It also gives accurate covalent bond lengths and atomization energies. Hence the functional proposed in this work is a computationally inexpensive electronic structure tool of broad applicability

Generation of basis sets for accurate molecular calculations: Application to helium atom and dimer

A new approach for basis set generation is reported and tested in helium atom and dimer. The basis sets thus computed, named sigma, range from DZ to 5Z and consist of the same composition as Dunning basis sets but with a different treatment of contractions. The performance of the sigma sets is analyzed for energy and other properties of He atom and He dimer, and the results are compared with those obtained with Dunning and ANO basis sets. The sigma basis sets and their extended versions up to triple augmented provide better energy values than Dunning basis sets of the same composition, and similar values to those attained with the currently available ANO. Extrapolation to complete basis set of correlation energy is compared between the sigma basis sets and those of Dunning, showing the better performance of the former in this respec

A Novel Approach to Design an Integrated Antenna-Battery System

In this study, an integrated antenna-battery was explored. Studying the systems separately allowed information to be obtained relating to the materials' performance and feasibility of an integrated system. Conducting polymers are promising in modern day lithium ion batteries. With high electrical conductivity as well as good ionic conductivity, they are now becoming more widely used. Here, we present a study of a co-block polymer (PEDOT-PEG) in which a polymer with high electrical conductivity is linked to a polymer with lithium ion conductivity, using a combination of atomistic simulations and experiments. Simulations showed that the diffusion and ionic conductivity for PEDOT-PEG agreed well with experiments. A trend was identified as a function of lithium salt concentration, in which the ionic conductivity decreased with increasing concentration. This was identified to be down to the significant ion pairing occurring in the system between lithium and the counterion. Requirements for the antenna were the ability to be mounted easily onto a battery substrate without a significant loss in efficiency and bandwidth. Studies were undertaken in which a slot dipole antenna was modified so as to incorporate properties more closely associated with battery materials i.e. permittivity and dielectric loss. An ultra-thin Mylar prototype was also synthesised and mounted onto a variety of surfaces, to assess how the antenna performed in different environments. Results for the antenna showed usable bandwidths and efficiencies when the antenna structure was modified to closely resemble a solid state battery. Despite a reduction seen in certain cases, these losses were not significant, and showed promise with regards to designing an integrated system. The Mylar prototype showed a good match between simulation and experiment in free space and when mounted on surfaces such as polymers, indicating that an ultra-thin antenna-battery is feasible

Efficient methods for the quantum chemical treatment of protein structures: The effects of London-dispersion and basis-set incompleteness on peptide and water-cluster geometries

We demonstrate how quantum chemical Hartree-Fock (HF) or density functional theory (DFT) optimizations with small basis sets of peptide and water cluster structures are decisively improved if London-dispersion effects, the basis-set-superposition error (BSSE), and other basis-set incompleteness errors are addressed. We concentrate on three empirical corrections to these problems advanced by Grimme and co-workers that lead to computational strategies that are both accurate and efficient. Our analysis encompasses a reoptimized version of Hobza's P26 set of tripeptide structures, a new test set of conformers of cysteine dimers, and isomers of the water hexamer. These systems reflect features commonly found in protein crystal structures. In all cases, we recommend Grimme's DFT-D3 correction for London-dispersion. We recommend usage of large basis sets such as cc-pVTZ whenever possible to reduce any BSSE effects and, if this is not possible, to use Grimme's gCP correction to account for BSSE when small basis sets are used. We demonstrate that S-S and C-S bond lengths are very prone to basis-set incompleteness and that polarization functions should always be used on S atoms. At the double-ζ level, the PW6B95-D3-gCP DFT method combined with the SVP and 6-31G* basis sets yields accurate results. Alternatively, the HF-D3-gCP/SV method is recommended, with inclusion of polarization functions for S atoms only. Minimal basis sets offer an intriguing route to highly efficient calculations, but due to significant basis-set incompleteness effects, calculated bond lengths are seriously overestimated, making applications to large proteins very difficult, but we show that Grimme's newest HF-3c correction overcomes this problem and so makes this computational strategy very attractive. Our results provide a useful guideline for future applications to the optimization, quantum refinement, and dynamics of large proteins. © 2013 American Chemical Society

Ab initio and force field investigations of physical hydrogen adsorption in Zeolitic Imidazole Frameworks

Recent theoretical calculations and experiments have considered that metal-organic frameworks are promising for storing molecular hydrogen (H2). Optimizing the geometry and the interaction energy of storing for enormous H2 storage is of great current interest. In this work, we used specific category of MOFs, Zeolitic Imidazole Frameworks (ZIFs). We carried out calculations through high-accuracy electronic structure calculations (MP2, CCSD and CCSD(T)) levels of theory, with controlled errors. Also we established and calibrated a computational protocol for accurately predicting the binding energy and structure of weakly bound complexes. Then, we applied the protocol to a number of models for metal-organic frameworks. For example, we have built many systems of noncovalently bound complexes [H2…benzene, H2….imidazole, CO…. imidazole, N2… imidazole, NH3…imidazole and H2O …imidazole] and we have optimized geometries of these systems through calculating numerical gradients at MP2/CP level and LMP2 level of theory and extrapolated from aug-cc-PVTZ and aug-cc-PVQZ basis set to evaluate the binding energy by using Hobza's scheme to obtain correct interaction energies. We found that NH3 and H2O with imidazole prefer to form hydrogen bonds rather than physical adsorption (London dispersion force). Also, the perpendicular position of hydrogen has the lowest potential energy surface, while the parallel hydrogen position has the highest potential energy surface. We have confirmed that by using a high level of basis set at MP2 such as ccpVXZ (x= Q, 5, 6) and aug-cc-pVXZ (x=D, T, Q, 5, 6), and by using the same basis sets at CCSD and CCSD(T) as the high level of theory. Also, it is clear from these results that the binding energies are sensitive to improvement of the size of basis sets. In terms of applying Hobza's scheme to obtain correct interaction energies, we found that this scheme CCSD(T)/ [34] = MP2/ [34] + (CCSD(T)/ [23] – MP2 [23]) achieved the highest accurate of interaction energy for CO...imidazole. On the other hand, this scheme CCSD(T)/ [34] = MP2/ [34] + [CCSD(T)/AVDZ– MP2/AVDZ] produced the highest accurate of interaction energy for H2...imi, N2…imi and H2…Benzene. Regarding to Basis Set Superposition Error (BSSE) and counterpoise examination (CP), Ab initio and Force field investigations of physical hydrogen adsorption in Zeolitic Imidazole Frameworks we found that the MP2/CP and LMP2 methods yield very similar results at the basis set limit and the convergence of MP2 and LMP2 with increasing size of basis sets is different since the BSSE in LMP2 is reduced. Furthermore, we found that the extrapolation to the CBS limit cannot offer an alternative to the counterpoise correction where the differences in the values of bending energies are large so we need to use both techniques together to overcome the BSSE problem. Then to confirm our result regard to the potential energy surface, we calculated corresponding potential energy surfaces using several popular force fields potential, and compare critically with best ab initio results, where we focused on the adsorption of H2 on imidazole as the organic linker in ZIFs. We carried out ab initio calculations at the MP2/CCSD(T) levels with different basis sets, basis set extrapolation and Lennard-Jones potential for the three directions X, Y and Z for 294 positions of H2. Also, we have fitted ab initio binding energy at the MP2/CCSD(T) levels with different basis set and basis set extrapolation to Lennard-Jones (12-6 LJ) binding energy by applying the nonlinear least squares method. Then we estimated the fitted binding energy using Hobza’s schemes to reduce the errors. We found that the 12-6 LJ formula produced unreasonable fit for ab initio calculated potential energy surface PES, for both the equilibrium and attractive regions, to improve this fitting, we found the good fit is only achieved by the exponential formula of repulsion region. It is hoped that this study could facilitate the search for a “good” application to store the H2 molecule conveniently and safely