Marc Brüssel

What can clusters tell us about the bulk?: Peacemaker: Extended quantum cluster equilibrium calculations

Abstract The quantum cluster equilibrium theory and its implementation into the freely available Peacemaker program are described in this paper. The important equations of the quantum cluster equilibrium with a constant mean field potential and an excluded volume are derived. The scheme of the consecutive iterations to obtain both natural variables (volume and particle number) is presented, as well. Further, we describe the implemented method to solve the polynomials arising in each iteration. Additionally, we deal with the question: Which kinds of particles are important for a physically meaningful description of a system. In order to arrive at a systematic classification of these particles, we discuss the important term of a cluster motif. We propose how to determine whether a particle is important for the investigated system and describe the implementation of this method into Peacemaker . Finally, in a case study the thermodynamic properties computed with Peacemaker and the corresponding experimental data are compared with each other.

Binary systems from quantum cluster equilibrium theory.

An extension of the quantum cluster equilibrium theory to treat binary mixtures is introduced in this work. The necessary equations are derived and a possible implementation is presented. In addition an alternative sampling procedure using widely available experimental data for the quantum cluster equilibrium approach is suggested and tested. An illustrative example, namely, the binary mixture of water and dimethyl sulfoxide, is given to demonstrate the new approach. A basic cluster set is introduced containing the relevant cluster motifs. The populations computed by the quantum cluster equilibrium approach are compared to the experimental data. Furthermore, the excess Gibbs free energy is computed and compared to experiments as well.

Comparison of Free Energy Surfaces Calculations from Ab Initio Molecular Dynamic Simulations at the Example of Two Transition Metal Catalyzed Reactions

We carried out ab initio molecular dynamic simulations in order to determine the free energy surfaces of two selected reactions including solvents, namely a rearrangement of a ruthenium oxoester in water and a carbon dioxide addition to a palladium complex in carbon dioxide. For the latter reaction we also investigated the gas phase reaction in order to take solvent effects into account. We used two techniques to reconstruct the free energy surfaces: thermodynamic integration and metadynamics. Furthermore, we gave a reasonable error estimation of the computed free energy surface. We calculated a reaction barrier of ΔF = 59.5 ± 8.5 kJ mol−1 for the rearrangement of a ruthenium oxoester in water from thermodynamic integration. For the carbon dioxide addition to the palladium complex in carbon dioxide we found a ΔF = 44.9 ± 3.3 kJ mol−1 from metadynamics simulations with one collective variable. The investigation of the same reactions in the gas phase resulted in ΔF = 24.9 ± 6.7 kJ mol−1 from thermodynamic integration, in ΔF = 26.7 ± 2.3 kJ mol−1 from metadynamics simulations with one collective variable, and in ΔF = 27.1 ± 5.9 kJ mol−1 from metadynamics simulations with two collective variables.

Theoretical Investigation of Solvent Effects and Complex Systems: Toward the calculations of bioinorganic systems from ab initio molecular dynamics simulations and static quantum chemistry

Abstract This mini review contains an overview of particular calculations on solvent effects and complexity, which can arise in bioinorganic chemistry. The major focus is set on the theoretical tool of ab initio molecular dynamics simulations. We provide a brief and simple introduction into the methodology in order to be comprehensible for a reader, who is not familiar with this field. In the second part we want to show how the method can be applied in a useful way. This includes (a) analyzation of the wavefunction, (b) determination of reaction mechanism, (c) studying solvent effects, and (d) combining advanced electronic structure techniques, such as the explicit relativistic Douglas–Kroll–Hess method, with molecular dynamics simulations. In the last part a case study of model complexes for nitrogen fixation is introduced. Finally, solvent effects and wavefunction analysis from a static quantum chemical point of view are discussed.

A Theoretical and Experimental Chemist’s Joint View on Hydrogen Bonding in Ionic Liquids and Their Binary Mixtures

A combined experimental and theoretical approach including quantum chemistry tools and computational simulation techniques can provide a holistic description of the nature of the interactions present in ionic liquid media. The nature of hydrogen bonding in ionic liquids is an especially intriguing aspect, and it is affected by all types of interactions occurring in this media. Overall, these interactions represent a delicate balance of forces that influence the structure and dynamics, and hence the properties of ionic liquids. An understanding of the fundamental principles can be achieved only by a combination of computations and experimental work. In this contribution we show recent results shedding light on the nature of hydrogen bonding, for certain cases the formation of a three-dimensional network of hydrogen bonding, and its dynamics by comparing 1-ethyl-3-methylimidazolium based acetate, chloride and thiocyanate ionic liquids.A particularly interesting case to study hydrogen bonding and other interactions is the investigation of binary mixtures of ionic liquids of the type [cation1][anion1]/[cation1][anion2]. In these mixtures, competing interactions are to be expected. We present both a thorough property meta-analysis of the literature and new data covering a wide range of anions, i.e., mixtures of 1-ethyl-3-methylimidazolium acetate with either trifluoroacetate, tetrafluoroborate, methanesulfonate, or bis(trifluoromethanesulfonyl)imide. In most cases, ideal mixing behavior is found, a surprising result considering the multitude of interactions present. However, ideal mixing behavior allows for the prediction of properties such as density, refractive index, surface tension, and, in most cases, viscosity as function of molar composition. Furthermore, we show that the prediction of properties such as the density of binary ionic liquid mixtures is possible by making use of group contribution methods which were originally developed for less complex non-ionic molecules. Notwithstanding this ideal mixing behavior, several exciting applications are discussed where preferential solvation via hydrogen bonding gives rise to non-additive effects leading to performance improvements. The assessment of the excess properties and (1)H NMR spectroscopic studies provide information on these structural changes and preferential interactions occurring in binary mixtures of ionic liquid, that clearly support the conclusions drawn from the computational studies.

A one-parameter quantum cluster equilibrium approach.

The established quantum cluster equilibrium approach is further developed in this work. The equations are reformulated to result in a one-parameter expression, i.e., with one of two empirical parameters eliminated. Instead of a parametrized constant mean field interaction we present two further approaches using temperature dependent mean field functions. The suggested functions are assessed by means of two test systems, namely hydrogen fluoride and water which are investigated concerning their liquid phase properties as well as the phenomenon of evaporation. The obtained thermodynamic data are compared with each other for the different mean field functions including the conventional approach as well as to experimental data.