We present a joint time-dependent density-functional theory for the description of solute-solvent systems in time-dependent external potentials. Starting with the exact quantum-mechanical action functional for both electrons and nuclei, we systematically eliminate solvent degrees of freedom and thus arrive at coarse-grained action functionals that retain the highly accurate ab initio description for the solute and are, in principle, exact. This procedure allows us to examine approximations underlying popular embedding theories for excited states.

We present a systematic ab initio study of antiferrodistortive (AFD) order in Ruddlesden-Popper (RP) phases of strontium titanate, Sr1+n Ti n O3n+1, as a function of both compressive epitaxial strain and phase number n. We find all RP phases to exhibit AFD order under a significant range of strains, recovering the AFD order of bulk SrTiO3 as ∼1/n2. A Ginzburg-Landau Hamiltonian including interoctahedral interactions reproduces our ab initio results well, opening a pathway to understanding other nanostructured perovskite systems. © 2010 The American Physical Society.

Graphical processing units are now being used with dramatic effect to accelerate quantum chemistry calculations. However, early work exposed challenges involving memory bottlenecks and insufficient numerical precision. This research effort addresses those issues, proposing two new tools for accelerating matrix multiplications of arbitrary size where single-precision accuracy is not enough. © 2010, IEEE. All rights reserved.

We present a theoretical study of the intrinsic quality factor of the fundamental flexural vibration in a carbon nanotube and its dependence on temperature, radius, length, and tension. In particular, we examine three- and four-phonon decays of the fundamental flexural mode within quantized elasticity theory.

We present an accurate free-energy functional for liquid water written in terms of a set of effective potential fields in which fictitious noninteracting water molecules move. The functional contains an exact expression of the entropy of noninteracting molecules and thus provides an ideal starting point for the inclusion of complex intermolecular interactions which depend on the orientation of the interacting molecules.

We present a study of the local strain effects associated with vacancy defects in strontium titanate and report the first calculations of elastic dipole tensors and chemical strains for point defects in perovskites. The combination of local and long-range results will enable determination of x-ray scattering signatures that can be compared with experiments. We find that the oxygen vacancy possesses a special property-a highly anisotropic elastic dipole tensor which almost vanishes upon averaging over all possible defect orientations.

The Gordian knot of density-functional theories for classical molecular liquids remains finding an accurate free-energy functional in terms of the densities of the atomic sites of the molecules. Following Kohn and Sham, we show how to solve this problem by considering noninteracting molecules in a set of effective potentials. This shift in perspective leads to an accurate and computationally tractable description in terms of simple three-dimensional functions.

We identify the shortcomings of existing ab initio quantum chemistry calculations for the hyperfine couplings in the recently characterized azafullerene, C59N. Standard gaussian basis sets in the context of all-electron calculations are insufficient to resolve the spin density near the cores of the atoms. Using the projector augmented wave (PAW) method implemented on top of a standard pseudo-potential plane-wave density-functional framework, we compute significantly more accurate values for the Fermi contact interaction. © 2002 Elsevier Science B.V. All rights reserved.

Under certain circumstances, decreasing the dimensions of a material may lead to elastic or anelastic properties that diverge from bulk behavior. A distinction is made between elastic deformation, for which bond rearrangements are not required, and anelastic behavior, which involves reversible deformation due to defect motion. Elastic deformation (due to bond stretching) remains structure-insensitive down to near-atomic length scales, and only small deviations are expected (of the order of 10%).

The Peierls stress for a [111]-screw dislocation in bcc Tantalum is calculated using an embedded atom potential. More importantly, a method is presented which allows accurate calculations of the Peierls stress in the smallest periodic cells. This method can be easily applied to ab initio calculations, where only the smallest unit cells capable of containing a dislocation can be conviently used. The calculation specifically focuses on the case where the maximum resolved shear stress is along a 110-plane.