Structural, Dynamical, and Electronic Properties of Liquid Water: A Hybrid Functional Study

We study structural, dynamical, and electronic properties of liquid water through ab initio molecular dynamics (MD) simulations based on a hybrid functional which includes nonlocal van der Waals (vdW) interactions. The water dimer, the water hexamer, and two phases of ice are studied as benchmark cases. The hydrogen-bond energy depends on the balance between Fock exchange and vdW interactions. Moreover, the energetic competition between extended and compact structural motifs is found to be well described by theory provided vdW interactions are accounted for. Applied to the hydrogen-bond network of liquid water, the dispersion interactions favor more compact structural motifs, bring the density closer to the experimental value, and improve the agreement with experimental observables such as radial distribution functions. The description of the self-diffusion coefficient is also found to improve upon the combined consideration of Fock exchange and vdW interactions. The band gap and the band edges are found to agree with experiment within 0.1 eV.

Isobaric first-principles molecular dynamics of liquid water with nonlocal van der Waals interactions

Giacomo Francesco Leonardo Miceli, Alfredo Pasquarello

We investigate the structural properties of liquid water at near ambient conditions using first-principles molecular dynamics simulations based on a semilocal density functional augmented with nonlocal van der Waals interactions. The adopted scheme offers the advantage of simulating liquid water at essentially the same computational cost of standard semilocal functionals. Applied to the water dimer and to ice I-h, we find that the hydrogen-bond energy is only slightly enhanced compared to a standard semilocal functional. We simulate liquid water through molecular dynamics in the N pH statistical ensemble allowing for fluctuations of the system density. The structure of the liquid departs from that found with a semilocal functional leading to more compact structural arrangements. This indicates that the directionality of the hydrogen-bond interaction has a diminished role as compared to the overall attractions, as expected when dispersion interactions are accounted for. This is substantiated through a detailed analysis comprising the study of the partial radial distribution functions, various local order indices, the hydrogen-bond network, and the selfdiffusion coefficient. The explicit treatment of the van der Waals interactions leads to an overall improved description of liquid water. (c) 2015 AIP Publishing LLC.

American Institute of Physics2015

Structural and electronic properties of an abrupt 4H-SiC(0001)/SiO2 interface model: Classical molecular dynamics simulations and density functional calculations

Peter Broqvist, Fabien Devynck, Alfredo Pasquarello

Using a density functional approach, we study structural and electronic properties of the 4H(0001)-SiC/SiO2 interface. Through the sequential use of classical and ab initio simulation methods, we generate an abrupt model structure which describes the transition between crystalline SiC and amorphous SiO2 without showing any coordination defect. The first step in our generation procedure consists in identifying suitable interfacial bonding patterns which account for the bond density reduction across the interface. In the second step, the connection to amorphous SiO2 is achieved through classical molecular dynamics. The atomic positions are then relaxed within a generalized gradient approximation of density functional theory. The final model structure shows good structural parameters and an oxide density typical of amorphous SiO2. We investigate the electronic structure of the generated model interface through the local density of states obtained with hybrid density functionals. In our atomically abrupt interface model, the full oxide band gap is recovered at a distance of similar to 5 A from the interface. This extent is in good agreement with estimates derived from internal photoemission measurements and provides support for the abrupt nature of the interface. We obtained band offsets through two different procedures: by evaluating the local density of states and by aligning the band extrema through the local electrostatic potential. Band offsets calculated with various functionals are compared to experimental values. The best agreement is achieved for a hybrid functional in which a screened Coulomb potential is used in the Hartree-Fock exchange term. For this case, the calculated band offsets underestimate the experimental values by similar to 25%, but agree with experiment within a few percent when expressed with respect to the oxide band gap.

2007

Release kinetics of volatiles from smectite clay minerals

Pascal Clausen

The focus of this thesis was on the mechanisms of release of volatile molecules from smectite clay minerals. The weight loss was monitored by thermogravimetry, differential calorimetry, and mass spectrometry under isothermal conditions and under ramp temperature conditions. Modeling of the clay-volatile release systems was performed with the help of finite element method calculations at the macroscopic level and ab initio calculations at the molecular level. A first comparison between finite element method simulations and experimental data for the evaporation of bulk volatile liquids under a convective gas flow showed the calculations to be in good agreement with experiments. Results of the simulations were used to develop a semi-analytical model explaining the dependence of the evaporation on the total pressure, the carrier gas flow, the temperature, and material constants. In particular, its temperature dependence could be approximated to good accuracy by an Arrhenius-type equation derived from the semi-analytical model. Differential calorimetry measurements of the heats of vaporization showed equilibrium conditions at the surface of the liquids to be satisfied. The same approach was extended to the release of model volatiles (water, ethanol, ethyl acetate and toluene) from smectite clays. At high coverage, the release was found to be close to that for the bulk liquids. Its decrease with time followed the behaviour observed in the respective curves of the gas/condensed phase partition coefficients, the equilibrium desorption isotherms. Equilibrium condition at the surface of the sample was evidenced by a comparison between the measured heats of vaporization and the equilibrium desorption isotherms. The differences observed in the measured equilibrium desorption isotherms of the volatile on the smectite clays could be rationalized by the use of ab initio calculations of the binding of the volatiles on the surface of a sodium smectite clay. At low coverage, the differences could be attributed to their differences in binding energies with the clay counter ions, which could be explained in terms of the chemical nature of the interacting species. At high coverage, the differences could be related to the properties of the bulk liquids, because high coverage corresponds to high activities of the volatiles on the clay samples. The differences observed in the measured rates of release of the volatiles from the smectite clays could be rationalized by the use of finite element calculations and the semi-analytical model developed for the bulk liquids, taking as input parameters the measured equilibrium desorption isotherms, that determines the volatile gas phase concentration at the surface of the sample. The slower rates of release of ethanol and ethyl acetate from the smectite clay, compared to water, could be explained from their differences in their equilibrium desorption isotherms from the clay, though diffusion effects also played a minor role. However, the slower rate of release of toluene, compared to water, could only be explained by its slow diffusion in the smectite clay. Diffusion effects were found to be enhanced with an increase in the size of the aggregate, such that, in addition to the binding strength of the counter ions, the swelling capacity of the clay, allowing the clay to clump, was also found to be an important factor for the controlled release of volatiles from the clay. Ion-exchange of the clay with lithium cations was found to be optimal in terms of ionic strength of the cation and swelling capacity, as compared to other metallic cations and small organic cations. This clay modification was tested on the release of malodorous compounds determined in cat urine by mass spectrometry and sniffing experiments. The smectite clay modified by ion-exchange with lithium cations showed improved controlled release of volatile organic compounds found in cat urine, compared to the sodium smectite, and improved clumping capacity compared to the calcium smectite. The calcium smectite clay used by Nestlé Purina could be modified by column exchange with lithium cations in high quantities. However the potential toxicity of the lithium is unknown. Additional information was obtained by ab initio calculations on the interaction of water with the smectite. The calculations of the binding of one to six water molecules on the surface of a sodium clay showed the water molecules bind to the cation and form hydrogen bonds between each other and with the clay oxygens, leading to the formation of different configurations for the same number of water molecules on the clay surface. With an increase in the number of water molecules on the surface of the clay, intermolecular interaction between the water molecules became gradually more important than sodium-water interactions. The binding energy was also found to be dependent on the location of the charge substitution in the clay (tetrahedral or octahedral) and to the formation of hydrogen bonds between the water molecule and a basal oxygen linked to a tetrahedral replacement. With an increase in the number of water molecules bound to one counter ion, the sodium counter ion was found to move away from the clay surface. The dynamic calculations of water molecules in the clay interlayer showed both the water molecules and the cations to be mobile. The average diffusion coefficient of the water molecules was reduced compared to that of bulk water, due to restrictions imposed by both the clay platelets and the counter ions. The structure of the water molecules showed to be similar to that of bulk water with a preferential orientation of their hydroxyl group almost perpendicular to the clay surface. The difference in the state of order of the water molecules around the sodium cation was interpreted as a symptom of hysteresis as observed in the measured equilibrium sorption isotherm.