Structural, hydrogen bonding and dipolar properties of alkyl imidazolium-based ionic liquids: a classical and first-principles molecular dynamics study

Ionic liquids (ILs) feature a tailorable and wide range of structural, chemical and electronic properties that make this class of materials suitable to a broad variety of forefront applications in next-generation electronics. Yet, their intrinsic complexity call for special attention and experimental probes have still limitations in unraveling the interactions occurring both in the bulk IL and at the interface with the solid substrates used to build the devices. This works provides an atomistic insight into these fundamental interactions by molecular modeling to complement the information still not accessible to experiments. In particular, we shed some light on the nature of the chemical bonding, structure, charge distribution and dipolar properties of a series of alkyl-imidazolium-based ILs by a synergy of classical and first-principles molecular dynamics simulations. Special emphasis is given to the crucial issue of the hydrogen bond network formation ability depending either on the nature of the anion or on the length of the alkyl chain of the cation. The hydrogen bond strength is a fundamental indicator of the cohesive and ordering features of the ILs and, in this respect, might be exploited to foster a different behaviour of the IL used a bulk medium or when used in electronic devices

Free energy molecular dynamics simulations of pulsed-laser-irradiated SiO2: Si–Si bond formation in a matrix of SiO2

Recent experiments have shown that pure Si structures in a matrix of SiO2 can be formed by electron excitation techniques, with appealing applications in nanotechnology. Our ab initio simulations provide an insight into the underlying mechanism, showing that electron excitations weaken Si–O bonds in SiO2, dislodge O atoms and allow Si dangling bonds to reconstruct in stable Si–Si structures below the melting temperature. Differences in diffusivity of O (fast) and Si (slow) are shown to play a decisive role in the process

Role of van der Waals corrections in first principles simulations of alkali metal ions in aqueous solutions

By resorting to a novel implementation of the first-principles-based van derWaals correction (vdWC) based on maximally localized Wannier functions (MLWFs), we inspect its performance and assess its reliability for aqueous solutions of alkali metal ions. In our implementation of vdWCs, an efficient extrapolation scheme is introduced to allow for affordable first principles molecular dynamics avoiding the explicit recalculation of MLWFs at each step. We find that vdWCs, when added to the widely used revPBE gradient corrected functional, influence substantially both structural and dynamical properties of water molecules, with particular emphasis on the hydration shell of the alkali cations. These effects are more evident for strong structure-making and -breaking cationic species. Moreover, self-diffusion coefficients and reorientation correlation times of solvating water molecules change systematically, showing a trend in better agreement with experiments with respect to simulations neglecting the long-range dispersion contributions. (C) 2015 AIP Publishing LLC

Communication: Hydration structure and polarization of heavy alkali ions: A first principles molecular dynamics study of Rb+ and Cs+:

Hydration structure and polarization of Rb+ and Cs+ in liquid water at ambient conditions were studied by first principles molecular dynamics. Our systematic analysis of the relevant electronic structures, based on maximally localized Wannier functions, revealed that the dipole moment of H2O molecules in the first solvation shell of the ions slightly increases with increasing the atomic number. We also found that the polarization of heavy alkali ions, particularly Cs+, tends to stabilize a peculiar asymmetric hydration structure with relevant consequences in the extraction of the harmful Cs-137 resulting from nuclear wastes. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4742151

Structural properties of glassy Ge2Se3 from first-principles molecular dynamics

The structural properties of glassy Ge2Se3 were studied in the framework of first-principles molecular dynamics by using the Becke-Lee-Yang-Parr scheme for the treatment of the exchange-correlation functional in density functional theory. Our results for the total neutron structure factor and the total pair distribution function are in very good agreement with the experimental results. When compared to the structural description obtained for liquid Ge2Se3, glassy Ge2Se3 is found to be characterized by a larger percentage of fourfold coordinated Ge atoms and a lower number of miscoordinations. However, Ge-Ge homopolar bonds inevitably occur due to the lack of Se atoms available, at this concentration, to form GeSe4 tetrahedra. Focusing on the family of glasses GexSe1-x, the present results allow a comparison to be carried out in reciprocal and real space among three prototypical glassy structures. The first was obtained at the stoichiometric composition (glassy GeSe2), the second at a Se-rich composition (glassy GeSe4) and the third at a Ge-rich composition (glassy Ge2Se3). All networks are consistent with the “8 - N” rule, in particular, glassy GeSe4, which exhibits the highest degree of chemical order. The electronic structure of glassy Ge2Se3 has been characterized by using the Wannier localized orbital formalism. The analysis of the Ge environment shows the presence of dangling, ionocovalent Ge-Se, and covalent bonds, the latter related to Ge-Ge connections. DOI: 10.1103/PhysRevB.86.224201This work was granted access by GENCI (Grand Equipement National de Calcul Intensif) under allocation 2011095071 to the HPC resources of CCRT/CINES/IDRI

The structure of liquid GeSe revisited: A first principles molecular dynamics study:

Early first-principles molecular dynamics results on liquid GeSe were characterized by shortcomings in the description of Ge-Ge (and to a lesser extent Se-Se) short range correlations. In that case the exchange-correlation functional adopted was the one devised by Perdew and Wang (PW91). In the search of improvements in the atomic-scale modelling of this liquid, we have produced new sets of data by employing two different schemes for the exchange-correlation part within the density functional theory approach. The two functionals selected are those proposed by Becke, Lee, Yang, and Parr (BLYP) and by Perdew, Burke, and Ernzerhof (PBE). The PBE results turned out to be quite similar to the PW91 ones. The BLYP results feature instead a better account of the Ge-Ge first shell of neighbors, correctly exhibiting two clear maxima separated by a deep minimum. Due to the increase in the number of the tetrahedral structural units, the atomic mobility of Ge and Se atoms in the network is reduced with respect to the PW91 case. This brings the diffusion coefficients of the two species down to values close to those of liquid Ge2Se3 and liquid GeSe2. (C) 2013 AIP Publishing LLC

Structural properties of liquid Ge₂Se₃: A first-principles study

The structural properties of liquid Ge2Se3 were investigated by first-principles molecular dynamics using the Becke-Lee-Yang-Parr scheme for the treatment of the exchange-correlation functional in density functional theory. Our data for the total neutron structure factor and the total pair-distribution function are in excellent agreement with the experimental results. The structure is made predominantly (similar to 61%) from units comprising fourfold coordinated Ge atoms in the form of Ge-GeSe3 or Ge-Se-4 motifs, but there is also a large variety of motifs in which Ge and Se are not fourfold and twofold coordinated, respectively. The miscoordinated atoms and homopolar bonds lead to a highly perturbed tetrahedral network, as reflected by diffusion coefficients that are larger than in the case of liquid GeSe2. The network does, nevertheless, exhibit intermediate range order which is associated with the Ge-Ge correlations and which manifests itself by a first sharp diffraction peak in the total neutron structure factor. The evolution of the properties of Ge-x Se1-x liquids (0 <= x <= 1) with composition is discussed.This work was granted access by GENCI (Grand Equipement National de Calcul Intensif) under allocation 2011095071 to the HPC resources of CCRT/CINES/IDRIS

Charge localization in DNA fibers.

We study by first-principles molecular dynamics the mechanism of electron hole (positive charge) localization in a laboratory realizable radical cation Z DNA crystal. We find that at room temperature structural deformation does not provide an efficient localization mechanism. Instead, we find evidence for the importance of changes in the protonation state for stabilizing the radical defect