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Vibrational properties of solids can be efficiently computed in a framework that combines density-functional theory and perturbation theory, called density-functional perturbation theory (DFPT). Recently, we have formulated DFPT for the projector-augmented wave (PAW) methodology, and we present a brief account of this work. The large effect of spin-orbit coupling on vibrational properties of Bi, Pb, and lead chalcogenides (PbS, PbSe, and PbTe) is also presented. [ABSTRACT FROM AUTHOR]

Using first-principles calculations based on a variational density functional perturbation theory, we investigate the lattice dynamics of solid solutions of barium and strontium titanates. Averaging the information available for the related pure compounds yields results equivalent to those obtained within the virtual crystal approximation, providing frequencies which are a good approximation to those computed for a (111) ordered supercell. Using the same averaging technique we report the evolution of the ferroelectric and antiferrodistortive instabilities with composition. [ABSTRACT FROM AUTHOR]

A wide variety of minerals exhibit ordered crystalline phases in which an underlying periodic structure is modulated by small atomic displacements with a wavelength incommensurate with this periodicity. We describe our recent efforts to use state-of-the-art first-principle calculations for the study of the microscopic mechanisms governing the existence of such phases and their transitions. A database containing more than one hundred incommensurate phases, available on the Web, has been constructed, and representative materials (PbO, AuTe[sub 2], K[sub 2]SO[sub 4], …) have been selected for our purpose. The ABINIT software project, thanks to which we can compute and analyze interatomic force constants and lattice instabilities, is described. We present preliminary results for the representative materials, discuss the difficulties that we face, and contrast them with those encountered in the study of ferroelectric materials. [ABSTRACT FROM AUTHOR]