A method of calculating surface energies for asymmetric slab models.

Many essential chemical processes, such as adsorption and catalysis, take place at the surface of a solid material. Hence, accurately determining the energy of a solid surface provides crucial information about the material's potential utility for such processes. The standard method of calculating surface energy yields good approximations for solids that, upon cleavage, expose identical surface terminations (symmetric slabs) but suffers critical shortcomings when applied to the multitude of materials that expose atomically different terminations (asymmetric slabs) due to the inaccurate assumption that the two terminations exhibit exactly the same energy.

The Stability of a Mixed-Phase Barium Cerium Iron Oxide under Reducing Conditions in the Presence of Hydrogen.

Metal oxide perovskite materials show promise for use as hydrogen separation membranes, but metal oxides can dehydrate in the presence of hydrogen to the point of decomposition. The stability of a material in the presence of hydrogen is necessary for an effective hydrogen separation membrane. The stability of a mixed phase metal oxide perovskite (BaCeFeO-BaCeFeO) was investigated using first-principles thermodynamics calculations based on density functional theory to examine the possible reduction processes on the surface of the material.

What's the gap? A possible strategy for advancing theory, and an appeal for experimental structure data to drive that advance.

There is substantial demand for theoretical/computational tools that can produce correct predictions of the geometric structure and band gap to accelerate the design and screening of new materials with desirable electronic properties. DFT-based methods exist that reliably predict electronic structure given the correct geometry. Similarly, when good spectroscopic data are available, these same methods may, in principle, be used as input to the inverse problem of generating a good structural model.

Investigation of net unidirectional ring shuttling in a chemically fueled [2]catenane.

Switchable rotaxanes and catenanes are environmentally responsive mechanically interlocked molecular architectures (MIMAs). Because of their ability to exhibit reversible and controllable motion in response to environmental stimuli, switchable rotaxanes and catenanes show promise for the advancement of nanoscale devices. Herein we present a study of the first 'autonomous' catenane-based motor (Wilson et al.

Unraveling the Semiconducting/Metallic Discrepancy in Ni(HITP).

Ni(2,3,6,7,10,11-hexaiminotriphenylene) is a π-stacked layered metal-organic framework material with extended π-conjugation that is analogous to graphene. Published experimental results indicate that the material is semiconducting, but all theoretical studies to date predict the bulk material to be metallic. Given that previous experimental work was carried out on specimens containing complex nanocrystalline microstructures and the tendency for internal interfaces to introduce transport barriers, we apply DFT to investigate the influence of internal interface defects on the electronic structure of Ni(HITP).

Application of semiempirical electronic structure theory to compute the force generated by a single surface-mounted switchable rotaxane.

Herein we report a study of the switchable [3]rotaxane reported by Huang et al. (Appl Phys Lett 85(22):5391-5393, 1) that can be mounted to a surface to form a nanomechanical, linear, molecular motor. We demonstrate the application of semiempirical electronic structure theory to predict the average and instantaneous force generated by redox-induced ring shuttling.

Empirical correction for PM7 band gaps of transition-metal oxides.

A post-calculation correction is established for PM7 band gaps of transition-metal oxides. The correction is based on the charge on the metal cation of interest, as obtained from MOPAC PM7 calculations. Application of the correction reduces the average error in the PM7 band gap from ~3 eV to ~1 eV.

Insights into the physical chemistry of materials from advances in HAADF-STEM.

The observation that, "New tools lead to new science"[P. S. Weiss, ACS Nano.

Role of proton hopping in surface charge transport on tin dioxide as revealed by the thermal dependence of conductance.

The presence of water on an oxide surface can dramatically alter its electrical properties with important consequences for electrical measurements by scanning probe microscopy, and for the use of semiconducting oxides in sensing applications. Here, the thermal dependence of the conductance of tin dioxide is interpreted by combining semiconductor equilibrium carrier statistics with a proton hopping mechanism. First, the functional form of this charge transport model is fit to experimental conductance data for tin dioxide.

Prediction of Charge Mobility in Amorphous Organic Materials through the Application of Hopping Theory.

The application of hopping theory to predict charge (hole) mobility in amorphous organic molecular materials is studied in detail. Application is made to amorphous cells of N,N'-diphenyl-N,N'-bis-(3-methylphenylene)-1,1'-diphenyl-4,4'-diamine (TPD), 1,1-bis-(4,4'-diethylaminophenyl)-4,4-diphenyl-1,3,butadinene (DEPB), N4,N4'-di(biphenyl-3-yl)-N4,N4'-diphenylbiphenyl-4,4'-diamine (mBPD), N1,N4-di(naphthalen-1-yl)-N1,N4-diphenylbenzene-1,4-diamine (NNP), and N,N'-bis[9,9-dimethyl-2-fluorenyl]-N,N'-diphenyl-9,9-dimethylfluorene-2,7-diamine (pFFA). Detailed analysis of the computation of each of the parameters in the equations for hopping rate is presented, including studies of their convergence with respect to various numerical approximations.

Computational investigation of the role of counterions and reorganization energy in a switchable bistable [2]rotaxane.

Switchable bistable [2]rotaxanes, such as those of the Stoddart-Heath-type, show promise for the development of molecular electronic devices and functional prototypes have been demonstrated. Herein, one such switchable rotaxane system is studied computationally at the AM1-FS1 and DFT levels of theory. The results show that the computationally efficient AM1-FS1 method, (efficient relative to DFT) is capable of reliably predicting properties such as binding site preference and coconformational relative stabilities as well as the barrier to isomerization between the different coconformational states.

A New Empirical Correction to the AM1 Method for Macromolecular Complexes.

Modeling systems that are governed by van der Waals (dispersion) interactions using empirically corrected DFT methods is becoming increasingly popular due to the promise of a CCSD(T) level accuracy at the computational cost of DFT. Although, DFT methods are computationally efficient in comparison to the CCSD(T) method, currently, structural optimizations using DFT methods are generally only feasible for systems of less than a few hundred atoms. We seek a method applicable to macromolecular complexes.

Empirically corrected DFT and semi-empirical methods for non-bonding interactions.

Computational modeling of systems governed by non-bonded interactions, especially van der Waals (dispersion) interactions, is currently a difficult task, since many conventional quantum mechanical techniques neglect such interactions. For example, the popular semi-empirical and Hartree-Fock methods, as well as most DFT methods all neglect long-range dispersion. In attempt to model dispersion interactions at reduced computational expense, one approach is to add an empirical potential to one of the quantum mechanical techniques.

Decomposition of the factors that govern binding site preference in a multiple rotaxane.

A particularly interesting class of multiple rotaxanes consists of complexes where one long shaft threads two rings. If the shaft contains three or more potential binding sites for the rings, multiple co-conformations are possible. Such a complex is a molecular topological analogue to an abacus.

Theoretical Study of Binding Site Preference in [2]Rotaxanes.

Rotaxanes that can be switched between co-conformations by some external stimulus are of interest because the switching mechanism might be used to create molecular devices capable of producing useful work. Probably the most common approach to create a switchable rotaxane is to start with a rotaxane where the ring interacts more strongly with one of two possible binding sites along the shaft and then apply an external stimulus that weakens the binding interaction between the ring and the shaft at this site, thereby changing the binding site preference. We have investigated binding site preference in two rotaxanes and two pseudorotaxanes with electronic structure calculations at several levels of theory.

2-Phenylpyridine: To Twist or Not To Twist?

Density functional theory methods were used to investigate the structures associated with 2-phenylpyridine, ppy, and several of its electronic states. The structure of ppy has the aromatic rings twisted with respect to one another by ∼21°, which is about half the value found for biphenyl. In comparison with ppy, both the isoelectronic cation, ppyH(+), and anion, ppy(-), have larger twist angles.

Origin of co-conformational selectivity in a [3]rotaxane.

Co-conformational selectivity and structure-energy relationships in a [3]rotaxane are investigated with a recently developed multiple-sampling and statistical analysis procedure for modeling interlocked molecules and mechanical molecular devices. The results presented confirm the experimentally observed co-conformational selectivity. The theoretical calculations reveal that ring-ring interactions are very small and ring-shaft inter-component interactions decide the co-conformational preference.

Dopants adsorbed as single atoms prevent degradation of catalysts.

The design of catalysts with desired chemical and thermal properties is viewed as a grand challenge for scientists and engineers. For operation at high temperatures, stability against structural transformations is a key requirement. Although doping has been found to impede degradation, the lack of atomistic understanding of the pertinent mechanism has hindered optimization.

The effect of substituents on molecular electronic junctions.

Previous reports have demonstrated that molecular electronic junctions exhibiting negative differential resistance show a marked substituent effect. We show that this substituent effect correlates with stability of the anionic form of the junction molecule. A sufficiently stable anion gives rise to a double potential barrier to electron transport across the molecular junction.

Atomic scale mechanism of the transformation of gamma-alumina to theta-alumina.

gamma-alumina is known to transform to theta-alumina and finally to alpha-alumina upon thermal treatment with a catastrophic loss of porosity and catalytic activity. First-principles calculations were performed to investigate the atomic scale mechanism of the gamma- to theta-alumina transformation. The transformation pathways between the two different forms have been mapped out and identified as a sequence of Al cation migrations.