Spin-polarized start-up potential

The Coulomb and XC (exchange + correlation) potentials are computed from the fit approximation of the charge density (see Chapter 1.2).

The fit coefficients of this approximation for the first SCF cycle, needed to compute the first Fock matrix, are read from the fragment files: the start-up density is chosen as a sum-of-fragment-densities (fit approximations) and this combined density defines the initial potential.

In the SCF restart run the fit coefficients may be read from the attached TAPE21 file, see the key RESTART.