The MFDSCF program

Belongs to Dirac-Fock part.

This module performs the SCF procedure necessary to solve the Dirac-Fock equations. These equations are based on the average field formalism of Roothaan [1] (1960). The relativistic analogue [2] is given by:


Qmn = Jmn + Kmn
f = e / s a = (s(e-1))/(e(s-1))
e = #electrons
s = #spinors

One can use one or two open shells, both with their own fractional occupation number. This configurational-average open-shell formalism covers most of the molecular open-shell systems.

The SCF process is relatively slowly convergent due to the size of the basis sets and may show oscillatory behaviour due to wrong intermediate ordering of closed and open shell spinors in the initial stages of the process as can happen in non-relativistic SCF as well. To cope with these problems various accelerating techniques, of which DIIS[3] works best, and damping techniques are implemented.

Other methods available are overlap selection with previous vectors, selection of open shell spinors by their population and a three- or four-point extrapolation method[4].

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