Ultrafast Relaxation of Symmetry-Breaking Photo-Induced Atomic Forces.

We present a first-principles method for the calculation of the temperature-dependent relaxation of symmetry-breaking atomic driving forces in photoexcited systems. We calculate the phonon-assisted decay of the photoexcited force on the low-symmetry E_{g} mode following absorption of an ultrafast pulse in Bi, Sb, and As. The force decay lifetimes for Bi and Sb are of the order of 10 fs and in agreement with recent experiments, demonstrating that electron-phonon scattering is the primary mechanism relaxing the symmetry-breaking forces.

Direct Measurement of Anharmonic Decay Channels of a Coherent Phonon.

We report channel-resolved measurements of the anharmonic coupling of the coherent A_{1g} phonon in photoexcited bismuth to pairs of high wave vector acoustic phonons. The decay of a coherent phonon can be understood as a parametric resonance process whereby the atomic displacement periodically modulates the frequency of a broad continuum of modes. This coupling drives temporal oscillations in the phonon mean-square displacements at the A_{1g} frequency that are observed across the Brillouin zone by femtosecond x-ray diffuse scattering.

First-principles calculation of femtosecond symmetry-breaking atomic forces in photoexcited bismuth.

We present a first-principles method for the calculation of the polarization-dependent atomic forces resulting from optical excitation in a solid. We calculate the induced force driving the E(g) phonon mode in bismuth immediately after absorption of polarized light. When radiation with polarization perpendicular to the c axis is absorbed, the photoexcited charge density breaks the threefold rotational symmetry, leading to an atomic force component perpendicular to the axis.

Dispersion interactions and vibrational effects in ice as a function of pressure: a first principles study.

We present a first principles theoretical framework that accurately accounts for several properties of ice, over a wide pressure range. In particular, we show that, by using a recently developed nonlocal van der Waals functional and by taking into account hydrogen zero point motion, one can properly describe the zero temperature equation of state, the vibrational spectra, and the dielectric properties of ice at low pressure and of ice VIII, a stable phase between 2 and 60 GPa. While semilocal density functionals yield a transition pressure from ice XI to VIII that is overestimated by almost an order of magnitude, we find good agreement with experiments when dispersion forces are taken into account.

Investigation of Exchange Energy Density Functional Accuracy for Interacting Molecules.

We present a comparison of exchange-only interaction energies obtained using several standard exchange functionals in the generalized gradient approximation to Hartree-Fock results for interacting molecules. We observe that functionals with an enhancement factor using a 2/5 power dependence on the gradient of the density for large density gradients offer consistently better agreement with Hartree-Fock calculations than that of alternative functionals. We revisit the functional offering the closest agreement and recalculate it to include its exact large gradient dependence.