Abstract: Density functional theory calculations were used to model the electrochemical reduction of CO2 on various transition metals, in particular Cu(111) and Pt(111) surfaces. The minimum energy paths for sequential protonation by either Tafel or Heyrovsky mechanism were calculated using the nudged elastic band method for applied potentials comparable to those used in experimental studies, ranging from -0.7 V to -1.7 V. A mechanism for CO2 reduction on Cu(111) has been identified where the highest activation energy is 0.4 eV. On Pt(111) a different mechanism is found to be optimal but it involves a higher barrier, 0.7 eV. Hydrogen production is then a competing reaction with activation barrier of only 0.3 eV, while on Cu(111) hydrogen production has a barrier of 0.6 eV. These results are consistent with experimental findings where copper electrodes are found to lead to relatively high yield of CH4 while H2 forms almost exclusively at platinum electrodes. A detailed understanding of the mechanism of electrochemical reduction of CO2 to hydrocarbons can help design improved catalysts for this important reaction.

Abstract: We describe new developments and applications of the Real Space Multigrid (RMG) electronic structure family of codes. RMG uses real-space grids, a multigrid pre-conditioner, and subspace diagonalization to solve the Kohn-Sham equations. It is designed for use on massively parallel computers and has shown excellent scalability and performance, reaching 6.5 PFLOPS on 18k Cray compute nodes with 288k CPU cores and 18k GPUs. We discuss (i) New developments in parallel subspace diagonalization, which speeds of the diagonalization part of the calculations by a factor of three or more; and (ii) Linear-scaling quantum transport methodology, which enable calculations for several thousand atoms. As examples, we consider: (iii) Molecular sensors based on carbon nanotubes, with configurations based both on direct attachment (physisorption and chemisorption) and indirect functionalization via covalent and non-covalent linkers; and (iv) Electron transport in DNA and the effects of base-pair matching, solvent and counterions. All of these dramatically affect the conductivity of DNA strands, which explains the wide range of results observed experimentally. If time permits, we will also discuss (v) fully quantum simulations of solvated biomolecules, in which Kohn-Sham (KS) DFT is used to describe the biomolecule and its first solvation shells, while the orbital-free (OF) DFT is employed for the rest of the solvent. The OF part is fully O(N) and capable of handling 10^5 solvent molecules on current parallel supercomputers, while taking only ~10% of the total time. RMG is now an open source code, running on Linux, Windows and MacIntosh systems. The current release of the code may be downloaded at http://sourceforge.net/projects/rmgdft/. In collaboration with E. Briggs, Y. Li, B. Tan, M. Hodak, and W. Lu.

Abstract: Supported nanoparticle catalysts are ubiquitous in heterogeneous catalytic processes, and there is broad interest in their physical and chemical properties. However, global probes such as XAS and XPS generally reveal their ensemble characteristics, obscuring details of their fluctuating internal structure. We have previously shown [1] that a combination of theoretical and experimental techniques is needed to understand the intra-particle heterogeneity of these systems [2], and their changes under operando conditions [3]. For example, ab initio DFT/MD simulations revealed that the nanoscale structure and charge distribution are inhomogeneous and dynamically fluctuating over several time-scales, ranging from fast (200-400 fs) bond vibrations to slow fluxional bond breaking (>10 ps). In particular the anomalous behavior of the mean-square relative displacement is not static, but rather is driven by stochastic motion of the center of mass over 1-4 ps time-scales. The resulting large scale fluctuations are termed “dynamic structural disorder” (DSD) [2]. Moreover, the nanoparticles tend to exhibit a semi-melted cluster surface, which for alloy clusters can be atomically-segregated. Recent studies of CO- and H-covered Pt nanoclusters on C and SiO2 supports show a variety of spectral and structural trends as a function of temperature. DFT simulations show that adsorption drives local electronic structure changes that are responsible for the opposite energy shifts vs temperature, of the absorption edge and off-resonant emission line. Moreover, desorption results in local bond contraction, thus explaining the negative thermal expansion observed in XAS experiments. For example, upon single CO adsorption, the Pt-Pt bonds formed by coordinated Pt atoms are locally expanded by ~5%, with little change in the rest of the particle. Coordination also has a large effect on the net charge of the Pt atoms (Figure 1), with a net loss of charge upon adsorption. Finally, we show how high coverage inverts the charging structure of the cluster, turning the negative surface (positive interior) of the clean cluster to positive surface (negative interior) in the fully covered case. Supported by DOE grant DE-FG02-03ER15476, with computer support from DOE-NERSC. [1] F. D. Vila, J. J. Rehr, J. Kas, R. G. Nuzzo and A. I. Frenkel, Phys. Rev. B 78, 121404(R) (2008). [2] J. J. Rehr and F. D. Vila, J. Chem. Phys. 140, 134701 (2014). [3] F. D. Vila, J. J. Rehr, S. D. Kelly and S. R. Bare J. Phys. Chem. C 117, 12446 (2013).

Abstract: Amorphous ice, or amorphous solid water (ASW), is the most common form of ice in astrophysical environments and is believed to be the dominant com- ponent of comets, planetary rings and dust grains in interstellar molecu- lar clouds. The surface of ASW catalyzes chemical reactions in interstellar space ranging from H 2 to complex organic molecules, and a deeper under- standing of ASW is thus crucial for better models of chemical evolution in the universe. ASW is disordered and metastable with respect to crystalline hexagonal ice, and forms when water molecules are deposited on surfaces at temperatures below 140 K. However, the structure, morphology and for- mation mechanisms of ASW are poorly understood and have received much attention across many disciplines. Indeed, the structure and morphology of ASW depend sensitively on how it forms, where key parameters are temper- ature, deposition rate and deposition angle. While atomistic simulations of ASW are extremely challenging due to slow kinetics and the long timescales involved, state-of-the-art long timescale methods provide a possible means to study the atomistic mechanisms involved on relevant timescales. Here we will discuss atomistic simulations of the growth and long timescale evolution of ASW through the use of the adaptive kinetic Monte Carlo (AKMC) tech- nique coupled to different interaction potentials for water molecules. The influence of temperature and deposition parameters is studied in detail and compared to available experimental results. Our results elucidate the struc- ture and formation mechanisms of ASW under astrophysical environments and provide realistic structure models that can be used in further studies of chemical reactivity of ASW surfaces.