  Fully-nonlocal two-projector norm-conserving pseudopotentials are shown to be
compatible with a systematic approach to the optimization of convergence with
the size of the plane-wave basis. A new formulation of the optimization is
developed, including the ability to apply it to positive-energy atomic
scattering states, and to enforce greater continuity in the pseudopotential.
The generalization of norm-conservation to multiple projectors is reviewed and
recast for the present purposes. Comparisons among the results of all-electron
and one- and two-projector norm-conserving pseudopotential calculations of
lattice constants and bulk moduli are made for a group of solids chosen to
represent a variety of types of bonding and a sampling of the periodic table.
