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QCE and Peacemaker


If a molecule possesses many conformers and we are interested in how much a particular conformer contributes to a certain property we want to address weights to the molecules. This is usually done by Boltzmann weighting which applies for the same system size, i.e., different conformers of one monomer. If we increase the system by including different oligomers, e.g. compare the contribution of a dimer with that of a monomer, we are unable to apply Boltzmann factors. Instead we can use cluster weighting. As in Boltzmann weighting, models of statistical thermodynamics are combined with quantum chemically calculated molecules – here, clusters of different size or oligomers. With this approach we can describe neat liquids and their mixtures at non-zero temperature and pressure in the condensed and gaseous phase. Upon inclusion of clusters with different composition, e.g. a solute solvated
with different amounts of solvent molecules, concentration dependent properties are accessible as well. Self-consistent-field calculations lead to equilibrium populations of these clusters and thus an ensemble of different structural states is generated similar to molecular dynamics simulations.



Paper examples:

  • Cluster weighting in bulk phase vibrational circular dichroism (218)
    Gas and bulk phase vibrational circular dichroism (VCD) spectra usually display very different features due to the intermolecular interactions occurring in the bulk. While first attempts for the calculation of bulk phase VCD spectra from ab initio molecular dynamics simulations have been made, these methods are still computationally demanding. With our cluster weighting procedure, a mixing of different oligomer sizes is possible at affordable computational effort. Thus explicit solvation is intrinsically taken care of, however, without neglecting monomeric structures and different conformers which are of particular importance for the calculation of VCD spectra. By a constant increase of the oligomer sizes included, combined with a continuous truncation of low populated clusters, we are able to constantly improve the agreement between theory and experiment until we reach an overlap which allows a certain assignment of the absolute configuration based on an experimental bulk phase VCD spectrum.
    Keywords: Vibrational circular dichroism, cluster weighting, conformational heterogeneity
  • Mole-fraction-dependent acid dissociation (201)
    The prediction of concentration dependent proton transfer processes in liquids usually requires computationally demanding calculations (e.g. ab initio molecular dynamics in combination with free energy methods), which have to be repeated for every composition of interest and thus limit such methods to a few selected cases at a time. In this work we predict the mole-fraction dependent acid dissociation for formic and acetic acid dissolved in water over the whole range of acid concentration at low computational cost. Since the method is based on a few easily accessible quantum chemical calculations, it can in principle be performed for any acid in any medium. We observe that the acid strength increases in a complex rather than a simple way, reflecting the complicated interplay between the dissociated ions or conjugate bases available. Calculated ion concentrations meet the experimental conductivity maximum with good to excellent agreement making the prediction of ionization processes in highly concentrated solutions possible.
    Keywords: acid strength, conductivity maximum, acid-base reaction, proton transfer
  • Activity coefficients (200)
    Activity coefficients are extremely useful parameters that describe a mixture’s deviation from ideal behavior. If they are known, the mixture’s properties can be predicted. Here, activity coefficients have been derived for the first time from genetically generated and quantum chemically optimized clusters for three different binary mixtures. The excess Gibbs energy of mixing of the three model systems acetonitrile/benzene, methanol/ethanol, and acetone/chloroform, each representing a different type of mixture, can be successfully predicted. Fitting standard Redlich-Kister polynomials to the excess Gibbs energy of mixing obtained from the cluster weighting gives rise to a new method of obtaining activity coefficients theoretically.
    Keywords: Clusters, activity coefficients, mixtures
  • PEACEMAKER 2 (199)
    Our quantum cluster equilibrium code PEACEMAKER is an open source free software written in modern FORTRAN 2008 and licensed under the GNU GPL. It allows the user to calculate cluster weightings in the liquid and the gaseous phase at any desired pressure and temperature point, giving access to numerous thermodynamic potentials, such as free energies, enthalpies, entropies, or heat capacities. PEACEMAKER is in constant development, recent improvements including the possibility to treat anharmonic vibrations, the inclusion of a downhill-simplex optimization algorithm, and the extension from binary to multi-component systems.
    Keywords: Molecular clusters, liquid mixtures, quantum chemistry, statistical thermodynamics
  • Protic ionic liquids (190)
    Protic ionic liquids (PILs) have low vapor pressure, a vast liquid range, and a high ionic conductivity. Their proton conductivity makes them attractive fuel cell electrolytes. But a high proton activity may come at the price of decreased thermal stability. Here, we develop a cluster approach to predict the proton activity and thermal properties of PILs. A selection of alkylammonium-based protic ionic liquids are investigated on the basis of genetically generated and quantum chemically optimized clusters. Without any experimental input, boiling points can be predicted within an accuracy of 50 K, reaching excellent accuracy of 1 K for the well-studied ethylammonium nitrate. Vaporization enthalpies in the 100 kJ/mol range are predicted within an accuracy of 20 kJ/mol and can be mechanistically interpreted on a molecular level. We present the first theoretical approach to predict proton activities in protic ionic liquids, with results fitting well into the experimentally observed correlation.
    Keywords: Ionic liquids, proton activity, ionicity

List of papers: 201, 200, 199, 190, 187, 184, 171, 150, 127, 112, 100, 99, 75, 74, 65, 36