New ultrasoft pseudopotentials for the study of silicates

New ultra-soft pseudopotentials (USP's) for Si and O suitable for studies of silicate materials are reported. The performance of the new USP's in density functional calculations on a number of model systems is documented and compared both to observed properties and to those computed in highly converged all-electron calculations. The new USP's are significantly more reliable and accurate than those previously developed and widely distributed. With the new USP's computed structural parameters follow well established trends with regard to local and gradient corrected treatments of exchange and correlation. The correct order of stability of the alpha-quartz and sodalite structures is also reproduced although the energy difference is not in quantitative agreement with the all-electron calculations. A new Al USP is also generated and its performance in calculations on alpha-alumina documented. The importance of consistent treatments of exchange and correlation for core and valence electrons is established.