In the Dirac-Fock method, the form of the single particle functions
constructing 
was variationally optimized by selecting
expansion coefficients 
which minimized the electronic energy.
In CI methods, the variational CI energy is variationally optimized with
respect to the expansion coefficients 
.
With multi-configuration SCF methods, a limited number of excited
determinants is selected, and a CI wavefunction involving these determinants
is constructed.
The CI energy resulting from this wavefunction is then
minimized with respect to changes in both the expansion coefficients for
the spinors and the CI coefficients. Clearly this method can be extremely
expensive if the number of excited determinants included in the CI space is
large. A practical implementation of the Multi-Configuration Dirac-Fock
method has been presented by Jensen, Dyall, Saue and
Faegri[12]. These wavefunctions
are often employed to construct a set of single particle spinors which may
then be used to construct a CI wavefunction which employs a larger active
space.