The pseudopotentials may be composed of a local part, and a linear combination of nonlocal operators. These nonlocal projectors have "projector" form, $ \left| v \right> \left< v \right| $ (or, more generally speaking, $ \left| u \right> \left< v \right| $). These projectors are localized in real space – that is, the function $v$ has a finite support around the nucleus. This region where the projectors are localized should be small or else the computation time required to operate with them will be very large.
In practice, this localization is fixed by requiring the definition of the projectors to be
contained in a sphere of a certain radius. This radius is computed by making sure that the
absolute value of the projector functions, at points outside the localization sphere, is
below a certain threshold. This threshold is set by SpeciesProjectorSphereThreshold.
species/ps.F90 : 194
call parse_variable(namespace, 'SpeciesProjectorSphereThreshold', CNST(0.001), ps%projectors_sphere_threshold)