Abstract:
We present an ab initio inelastic quantum transport approach based on maximally localized Wannier functions. Electronic-structure properties are calculated with density-functional theory in a planewave basis, and electron-vibration coupling strengths and vibrational properties are determined with density-functional perturbation theory. Vibration-induced inelastic transport properties are calculated with non-equilibrium Green's function techniques, which are based on localized orbitals. For this purpose we construct maximally localized Wannier functions. Our formalism is applied to investigate inelastic transport in a benzene molecular junction connected to mono-atomic carbon chains. In this benchmark system the electron-vibration self-energy is calculated either in the self-consistent Born approximation or by lowest-order perturbation theory. It is observed that upward and downward conductance steps occur, which can be understood using multi-eigenchannel scattering theory and symmetry conditions. In a second example where the mono-atomic carbon chain electrode is replaced by a (3; 3) carbon nanotube, we focus on the non-equilibrium vibration populations driven by the conducting electrons using a semi-classical rate equation.

Abstract:
We characterize the response of isolated single- (SWNT) and multi-wall (MWNT) carbon nanotubes and bundles to static electric fields using first-principles calculations and density-functional theory. The longitudinal polarizability of SWNTs scales as the inverse square of the band gap, while in MWNTs and bundles it is given by the sum of the polarizabilities of the constituent tubes. The transverse polarizability of SWNTs is insensitive to band gaps and chiralities and is proportional to the square of the effective radius; in MWNTs the outer layers dominate the response. The transverse response is intermediate between metallic and insulating, and a simple electrostatic model based on a scale-invariance relation captures accurately the first-principles results. Dielectric response of non-chiral SWNTs in both directions remains linear up to very high values of applied field.

Abstract:
Continuum solvation methods can provide an accurate and inexpensive embedding of quantum simulations in liquid or complex dielectric environments. Notwithstanding a long history and manifold applications to isolated systems in open boundary conditions, their extension to materials simulations --- typically entailing periodic-boundary conditions --- is very recent, and special care is needed to address correctly the electrostatic terms. We discuss here how periodic-boundary corrections developed for systems in vacuum should be modified to take into account solvent effects, using as a general framework the self-consistent continuum solvation model developed within plane-wave density-functional theory [O. Andreussi et al. J. Chem. Phys. 136, 064102 (2012)]. A comprehensive discussion of real-space and reciprocal-space corrective approaches is presented, together with an assessment of their ability to remove electrostatic interactions between periodic replicas. Numerical results for zero-dimensional and two-dimensional charged systems highlight the effectiveness of the different suggestions, and underline the importance of a proper treatement of electrostatic interactions in first-principles studies of charged systems in solution.

Abstract:
We discuss a method for determining the optimally-localized set of generalized Wannier functions associated with a set of Bloch bands in a crystalline solid. By ``generalized Wannier functions'' we mean a set of localized orthonormal orbitals spanning the same space as the specified set of Bloch bands. Although we minimize a functional that represents the total spread sum_n [ _n - _n^2 ] of the Wannier functions in real space, our method proceeds directly from the Bloch functions as represented on a mesh of k-points, and carries out the minimization in a space of unitary matrices U_mn^k describing the rotation among the Bloch bands at each k-point. The method is thus suitable for use in connection with conventional electronic-structure codes. The procedure also returns the total electric polarization as well as the location of each Wannier center. Sample results for Si, GaAs, molecular C2H4, and LiCl will be presented.

Abstract:
We calculate the linear and non-linear susceptibilities of periodic longitudinal chains of hydrogen dimers with different bond-length alternations using a diffusion quantum Monte Carlo approach. These quantities are derived from the changes in electronic polarization as a function of applied finite electric field - an approach we recently introduced and made possible by the use of a Berry-phase, many-body electric-enthalpy functional. Calculated susceptibilities and hyper-susceptibilities are found to be in excellent agreement with the best estimates available from quantum chemistry - usually extrapolations to the infinite-chain limit of calculations for chains of finite length. It is found that while exchange effects dominate the proper description of the susceptibilities, second hyper-susceptibilities are greatly affected by electronic correlations. We also assess how different approximations to the nodal surface of the many-body wavefunction affect the accuracy of the calculated susceptibilities.

Abstract:
Room Temperature Ionic Liquids (RTILs) have attracted much of the attention of the scientific community in the past decade due the their novel and highly customizable properties. Nonetheless their high viscosities pose serious limitations to the use of RTILs in practical applications. To elucidate some of the physical aspects behind transport properties of RTILs, extensive classical molecular dynamics (MD) calculations are reported. Bulk viscosities and ionic conductivities of butyl-methyl-imidazole based RTILs are presented over a wide range of temperatures. The dependence of the properties of the liquids on simulation parameters, e.g. system size effects and choice of the interaction potential, is analyzed.

Abstract:
The electronic ground state of a periodic crystalline solid is usually described in terms of extended Bloch orbitals; localized Wannier functions can alternatively be used. These two representations are connected by families of unitary transformations, carrying a large degree of arbitrariness. We have developed a localization algorithm that allows one to iteratively transform the extended Bloch orbitals of a first-principles calculation into a unique set of maximally localized Wannier functions. We apply this formalism here to the case of cubic BaTiO3. The purpose is twofold. First, a localized-orbital picture allows a meaningful band-by-band decomposition of the whole Bloch band complex. In perovskites, these Wannier functions are centered on the atomic sites and display clearly a s, p, d, or hybrid character. Second, since the centers of the Wannier functions map the polarization field onto localized point charges, the ground state dielectric properties become readily available. We study the Born effective charges of the paraelectric phase of BaTiO3. We are able to identify not only the contributions that come from a given group of bands, but also the individual contributions from the ``atomic'' Wannier functions that comprise each of these groups.

Abstract:
The structural, dynamical, and thermodynamical properties of diamond, graphite and layered derivatives (graphene, rhombohedral graphite) are computed using a combination of density-functional theory (DFT) total-energy calculations and density-functional perturbation theory (DFPT) lattice dynamics at the GGA-PBE level. Overall, very good agreement is found for the structural properties and phonon dispersions, with the exception of the c/a ratio in graphite and the associated elastic constants and phonon dispersions. Both the C_33 elastic constant and the Gamma to A phonon dispersions are brought to close agreement with available data once the experimental c/a is chosen for the calculations. The thermal expansion, the temperature dependence of the elastic moduli and the specific heat have been calculated via the quasi-harmonic approximation. Graphite shows a distinctive in-plane negative thermal-expansion coefficient that reaches the minimum around room temperature, in very good agreement with experiments. Thermal contraction in graphene is found to be three times as large; in both cases, ZA acoustic modes are shown to be responsible for the contraction, in a direct manifestation of the membrane effect predicted by Lifshitz over fifty years ago.

Abstract:
We highlight the emergence of metallic states in two-dimensional transition-metal-dichalcogenide nanostructures -nanoribbons, islands, and inversion domain boundaries- as a widespread and universal phenomenon driven by the polar discontinuities occurring at their edges or boundaries. We show that such metallic states form one-dimensional wires of electrons or holes, with a free charge density that increases with the system size, up to complete screening of the polarization charge, and can also be controlled by the specific edge or boundary configurations, e.g. through chemisorption of hydrogen or sulfur atoms at the edges. For triangular islands, local polar discontinuities occur even in the absence of a total dipole moment for the island and lead to an accumulation of free carriers close to the edges, providing a consistent explanation of previous experimental observations. To further stress the universal character of these mechanisms, we show that polar discontinuities give rise to metallic states also at inversion domain boundaries. These findings underscore the potential of engineering transition-metal-dichalcogenide nanostructures for manifold applications in nano- and opto-electronics, spintronics, catalysis, and solar-energy harvesting.

Abstract:
The pi-stacking between aromatic oligomers has been extensively studied for many years, although the notion of exploiting this phenomenon as the driving force for molecular actuation has only recently emerged. In this work we examine with MP2 and Car-Parrinello ab initio calculations the actuation properties of a novel class of thiophene-based materials introduced by Swager et al.(Adv. Mater. 14, 368 (2002); JACS 125, 1142 (2003)). The chemical ingredients of the assembly, calix[4]arenes and oligothiophenes, are screened separately to characterize the actuation mechanisms and design optimal architectures. In particular, ab initio methods are used to study pi-stacking in mixed valence oligothiophene dimers, revealing strong interactions that can be turned on and off as a function of the electrochemical potential. We show how these interactions could be harnessed to achieve molecular actuation and investigate the response of an active unit in real time with first-principles molecular dynamics simulations.