Ivan Stich

Protonated hydrogen clusters

The effect of protonation of pure hydrogen clusters is investigated at low temperature using a combination of path integral simulations and first-principles density functional electronic structure calculations. These odd n Hn+ clusters are shown to lose the quantum-liquid properties of their unprotonated counterparts. The added proton gets trapped as a very localized and strongly bound H3+ impurity in the cluster core, surrounded by stable shells of more spatially delocalized solvating H2 molecules. The clusters are frozen with respect to the translational degrees of freedom, while the H2 ligands undergo large-amplitude rotations. The rotational delocalization is found to increase in successive solvation shells. The combination of translational rigidity and rotational floppiness, which is akin to plastic behavior in crystals, is a quantum induced phenomenon absent in the classical approximation for the nuclei.

Switching of functionalized azobenzene suspended between gold tips by mechanochemical, photochemical, and opto-mechanical means

Optical, purely mechanical, and combined opto-mechanical switching cycles of a molecular switch embedded in a metal junction are investigated using density functional theory and (excited state) ab initio molecular dynamics. The nanomechanical simulations are done on realistic models of gold electrode tips bridged by a single dithioazobenzene molecule. Comparison of different tip models shows that the nature of the tips affects switching processes both qualitatively and quantitatively. The study predicts that purely photochemical cis⇌trans switching cycles of suspended azobenzene bridges are mechanically hindered; combined opto-mechanical as well as purely mechanochemical forward and backward switching is, however, feasible.

Dynamical observation of the catalytic activation of methanol in zeolites

Abstract The adsorption of methanol in the zeolites chabazite, ferrierite and silicalite has been studied using first principles molecular dynamics simulations. The study includes an investigation of the effect of both loading and temperature. At low coverage, methanol forms a hydrogen bonded complex in chabazite and silicalite but proton transfer appears to be stable in ferrierite when the methanol is in the eight-ring side channel. As the loading increases the methoxonium cation becomes the stable species and a surprisingly large weakening of the C–O bond occurs due to dynamical effects which may explain the enhanced susceptibility of the methanol to nucleophilic attack.

Theoretical study of the structural evolution of small hydrogenated silicon clusters: Si6Hx

Abstract Density functional calculations were performed for the structural properties and energetics of small hydrogenated silicon clusters: Si6Hx (0 ⩽ x ⩽ 14). We find that the structures of Si6Hx can be classified into several distinct families in terms of the arrangement of silicon atoms. In particular, we find a series of structures which are intermediate between compact and tetrahedral atomic arrangements. Based on calculated formation energies we address the relative stability of the Si6Hx clusters.

Methanol in microporous materials from first principles

Abstract Zeolites are amongst some of the most important heterogeneous catalysts in use commercially today, combining acid–base catalysis due to the presence of Bronsted acid sites with shape selectivity resulting from the microporous environment [1] . Despite this, there is still a great deal of uncertainty concerning the mechanisms of many of the processes which are known to occur and the way in which the zeolite accelerates them. While much information has been obtained from experimental techniques, including infra-red spectroscopy and magic angle spinning NMR [2] , there is presently a need for models and reaction pathways to aid in their interpretation. Here theoretical methods are playing a major role in the field of microporous materials.

Ground and excited electronic states of azobenzene: A quantum Monte Carlo study

Large-scale quantum Monte Carlo (QMC) calculations of ground and excited singlet states of both conformers of azobenzene are presented. Remarkable accuracy is achieved by combining medium accuracy quantum chemistry methods with QMC. The results not only reproduce measured values with chemical accuracy but the accuracy is sufficient to identify part of experimental results which appear to be biased. Novel analysis of nodal surface structure yields new insights and control over their convergence, providing boost to the chemical accuracy electronic structure methods of large molecular systems.

Thermodynamics of Catalytic Formation of Dimethyl Ether from Methanol in Acidic Zeolites

We present a theoretical study of the formation of the first intermediate, dimethyl ether, in the methanol to gasoline conversion within the framework of an ab initio molecular dynamics approach. The study is performed under conditions that closely resemble the reaction conditions in the zeolite catalyst including the full topology of the framework. The use of the method of thermodynamic integration allows us to extract the free-energy profile along the reaction coordinate. We find that the entropic contribution qualitatively alters the free-energy profile relative to the total energy profile. Different transition states are found from the internal and free energy profiles. The entropy contribution varies significantly along the reaction coordinate and is responsible for stabilizing the products and for lowering the energy barrier. The hugely inhomogeneous variation of the entropy can be understood in terms of elementary processes that take place during the chemical reaction. Our simulations provide new insights into the complex nature of this chemical reaction.

Ground-state reconstruction of the Si(0 0 1) surface: symmetric versus buckled dimers

An extensive computational study is presented with the quest to investigate the nature of the ground-state geometry of the Si(0 0 1) surface, a subject of recent experimental controversy. We analyze for the first time in detail the possible sources of errors which would arise in any correlated calculation for a system size of interest here. For this purpose, we present a detailed analysis of the cluster model of the surface at the DFT and MCSCF level of theory. Estimates of errors arising from the use of pseudopotential, finite cluster size, and biased (method dependent) choice of ground-state geometry are given. The resulting error is estimated to be comparable to the energy scale of interest. On the other hand, the energy variation due to negative thermal expansion at low temperature is found to be qualitatively consistent with dimer symmetrization.

Interplay of the tip-sample junction stability and image contrast reversal on a Cu(111) surface revealed by the 3D force field.

Non-contact atomic force microscopy is used to measure the 3D force field on a dense-packed Cu(111) surface. An unexpected image contrast reversal is observed as the tip is moved towards the surface, with atoms appearing first as bright spots, whereas hollow and bridge sites turn bright at smaller tip-sample distances. Computer modeling is used to elucidate the nature of the image contrast. We find that the contrast reversal is essentially a geometrical effect, which, unlike in gold, is observable in Cu due to an unusually large stability of the tip-sample junction over large distances.

Quantum Monte Carlo Study of π-Bonded Transition Metal Organometallics: Neutral and Cationic Vanadium–Benzene and Cobalt–Benzene Half Sandwiches

We present accurate quantum Monte Carlo (QMC) calculations that enabled us to determine the structure, spin multiplicity, ionization energy, dissociation energy, and spin-dependent electronic gaps of neutral and positively charged vanadium-benzene and cobalt-benzene systems. From total/ionization energy, we deduce a sextet (quintet) state of neutral (cationic) vanadium-benzene systems and quartet (triplet) state of the neutral (cationic) cobalt-benzene systems. Vastly different energy gaps for the two spin channels are predicted for the vanadium-benzene system and broadly similar energy gaps for the cobalt-benzene system. For this purpose, we have used a multistage combination of techniques with consecutive elimination of systematic biases except for the fixed-node approximation in QMC. Our results significantly differ from the established picture based on previous less accurate calculations and point out the importance of high-level many-body methods for predictive calculations of similar transition metal-based organometallic systems.