VASP terminology

Exchange-correlation function

LDA: Local density approximation, using explicit analytic function of E_xc

GGA: correction to LDA at large magnitude of gradient in the number density. PBE-GGA, PW91-GGA, ..., attempts to use different form of E_xc

Pseudopotential

OPW: separation of core region and valance region. valance region is described by a smooth function

Ultrasoft Pseudo potential: even core region is described by a smooth function

PAW: correction to ultrasoft pseudopotential by considering the whole wave function to compensate the discrepancy between localized function and ultrasmooth function in the core region