Publications

Wills, Ann E.

Abstract not provided.

Proposed for publication in Physical Review Letters.

Wills, Ann E.

We design a density-functional-theory (DFT) exchange-correlation functional that enables an accurate treatment of systems with electronic surfaces. Surface-specific approximations for both exchange and correlation energies are developed. A subsystem functional approach is then used: an interpolation index combines the surface functional with a functional for interior regions. When the local density approximation is used in the interior, the result is a straightforward functional for use in self-consistent DFT. The functional is validated for two metals (Al, Pt) and one semiconductor (Si) by calculations of (i) established bulk properties (lattice constants and bulk moduli) and (ii) a property where surface effects exist (the vacancy formation energy). Good and coherent results indicate that this functional may serve well as a universal first choice for solid-state systems and that yet improved functionals can be constructed by this approach.

Wills, Ann E.

We present an exchange-correlation functional that enables an accurate treatment of systems with electronic surfaces. The functional is developed within the subsystem functional paradigm [1], combining the local density approximation for interior regions with a new functional designed for surface regions. It is validated for a variety of materials by calculations of: (i) properties where surface effects exist, and (ii) established bulk properties. Good and coherent results are obtained, indicating that this functional may serve well as universal first choice for solid state systems. The good performance of this first subsystem functional also suggests that yet improved functionals can be constructed by this approach.

Wills, Ann E.

Abstract not provided.

Carpenter, John H.; Desjarlais, Michael P.; Wills, Ann E.

Abstract not provided.

Carpenter, John H.; Robinson, Allen C.; Wills, Ann E.; Debusschere, Bert D.

Abstract not provided.

Carpenter, John H.; Robinson, Allen C.; Debusschere, Bert D.; Wills, Ann E.

Abstract not provided.

Wills, Ann E.

Abstract not provided.

Wills, Ann E.; Wixom, Ryan R.; Mattsson, Thomas M.

Abstract not provided.

Wills, Ann E.

Abstract not provided.

Robinson, Allen C.; Petney, Sharon P.; Drake, Richard R.; Weirs, Vincent G.; Adams, Brian M.; Vigil, Dena V.; Carpenter, John H.; Garasi, Christopher J.; Wong, Michael K.; Robbins, Joshua R.; Siefert, Christopher S.; Strack, Otto E.; Wills, Ann E.; Trucano, Timothy G.; Bochev, Pavel B.; Summers, Randall M.; Stewart, James R.; Ober, Curtis C.; Rider, William J.; Haill, Thomas A.; Lemke, Raymond W.; Cochrane, Kyle C.; Desjarlais, Michael P.; Love, Edward L.; Voth, Thomas E.; Mosso, Stewart J.; Niederhaus, John H.

Abstract not provided.

Wills, Ann E.; Thompson, Aidan P.; Raman, Sumathy R.

We establish an atomistic view of the high- and low-temperature phases of iron/steel as well as some elements of the phase transition between these phases on cooling. In particular we examine the 4 most common orientation relationships between the high temperature austen- ite and low-temperature ferrite phases seen in experiment. With a thorough understanding of these relationships we are prepared to set up various atomistic simulations, using tech- niques such as Density Functional Theory and Molecular Dynamics, to further study the phase transition, in particular, quantities needed for Phase Field Modeling, such as the free energies of bulk phases and the phase transition front propagation velocity.

Carpenter, John H.; Robinson, Allen C.; Debusschere, Bert D.; Wills, Ann E.; Drake, Richard R.; Rider, William J.

Abstract not provided.

Wills, Ann E.; Rider, William J.

Abstract not provided.

Wills, Ann E.

Density Functional Theory (DFT) based Equation of State (EOS) construction is a prominent part of Sandia’s capabilities to support engineering sciences. This capability is based on augmenting experimental data with information gained from computational investigations, especially in those parts of the phase space where experimental data is hard, dangerous, or expensive to obtain. A key part of the success of the Sandia approach is the fundamental science work supporting the computational capability. Not only does this work enhance the capability to perform highly accurate calculations but it also provides crucial insight into the limitations of the computational tools, providing high confidence in the results even where results cannot be, or have not yet been, validated by experimental data. This report concerns the key ingredient of projector augmented-wave (PAW) potentials for use in pseudo-potential computational codes. Using the tools discussed in SAND2012-7389 we assess the standard Vienna Ab-initio Simulation Package (VASP) PAWs for Molybdenum.

Shan, Tzu-Ray S.; Thompson, Aidan P.; Wills, Ann E.

Abstract not provided.

Carpenter, John H.; Robinson, Allen C.; Debusschere, Bert D.; Wills, Ann E.

Abstract not provided.

Carpenter, John H.; Robinson, Allen C.; Debusschere, Bert D.; Wills, Ann E.

Abstract not provided.

Wills, Ann E.

Abstract not provided.

Modine, N.A.; Wright, Alan F.; Muller, Richard P.; Sears, Mark P.; Wills, Ann E.; Desjarlais, Michael P.

A finite temperature version of 'exact-exchange' density functional theory (EXX) has been implemented in Sandia's Socorro code. The method uses the optimized effective potential (OEP) formalism and an efficient gradient-based iterative minimization of the energy. The derivation of the gradient is based on the density matrix, simplifying the extension to finite temperatures. A stand-alone all-electron exact-exchange capability has been developed for testing exact exchange and compatible correlation functionals on small systems. Calculations of eigenvalues for the helium atom, beryllium atom, and the hydrogen molecule are reported, showing excellent agreement with highly converged quantumMonte Carlo calculations. Several approaches to the generation of pseudopotentials for use in EXX calculations have been examined and are discussed. The difficult problem of finding a correlation functional compatible with EXX has been studied and some initial findings are reported.

Carpenter, John H.; Robinson, Allen C.; Debusschere, Bert D.; Wills, Ann E.

Abstract not provided.

Wills, Ann E.

We develop a specialized treatment of electronic surface regions which, via the subsystem functional approach [1], can be used in functionals for self-consistent density-functional theory (DFT). Approximations for both exchange and correlation energies are derived for an electronic surface. An interpolation index is used to combine this surface-specific functional with a functional for interior regions. When the local density approximation (LDA) is used for the interior region, the end result is a straightforward density-gradient dependent functional that shows promising results. Further improvement of the treatment of the interior region by the use of a local gradient expansion approximation is also discussed.

Wills, Ann E.; Wixom, Ryan R.; Mattsson, Thomas M.

Density Functional Theory (DFT) has over the last few years emerged as an indispensable tool for understanding the behavior of matter under extreme conditions. DFT based molecular dynamics simulations (MD) have for example confirmed experimental findings for shocked deuterium, enabled the first experimental evidence for a triple point in carbon above 850 GPa, and amended experimental data for constructing a global equation of state (EOS) for water, carrying implications for planetary physics. The ability to perform high-fidelity calculations is even more important for cases where experiments are impossible to perform, dangerous, and/or prohibitively expensive. For solid explosives, and other molecular crystals, similar success has been severely hampered by an inability of describing the materials at equilibrium. The binding mechanism of molecular crystals (van der Waals forces) is not well described within traditional DFT. Among widely used exchange-correlation functionals, neither LDA nor PBE balances the strong intra-molecular chemical bonding and the weak inter-molecular attraction, resulting in incorrect equilibrium density, negatively affecting the construction of EOS for undetonated high explosives. We are exploring a way of bypassing this problem by using the new Armiento-Mattsson 2005 (AM05) exchange-correlation functional. The AM05 functional is highly accurate for a wide range of solids, in particular in compression. In addition, AM05 does not include any van der Waals attraction, which can be advantageous compared to other functionals: Correcting for a fictitious van der Waals like attraction with unknown origin can be harder than correcting for a complete absence of all types of van der Waals attraction. We will show examples from other materials systems where van der Waals attraction plays a key role, where this scheme has worked well, and discuss preliminary results for molecular crystals and explosives.

Wills, Ann E.; Mattsson, Thomas M.

Abstract not provided.

Wixom, Ryan R.; Mattsson, Thomas M.; Wills, Ann E.

Abstract not provided.