Commercial CFDRC software (http://www.cfdrc.com/~cfdplasma) is used to self-consistently simulate the plasma of the positive column (PC) of a dc discharge in argon. The software allows simulations in an arbitrary 3D geometry by using Poisson's equation for the electric potential and fluid equations for the heavy components and by solving a nonlocal kinetic equation for electrons. It is shown that, in calculating the electron distribution function, the local approximation is almost always inapplicable under real conditions of a diffuse PC usually met in practice (pR < (5-10) cm Torr). The influence of metastable atoms, which can substantially affect the parameters of the PC plasma, is considered. It is shown that superelasic collisions play an important role in enriching the fast component of the electron distribution function and that the Penning ionization can result in an ascending volt-ampere characteristic of the positive column.