The four-component wavefunctions of the Dirac-Hartee-Fock method
present a practical barrier for the quantum chemical investigator.
Methods which decouple the positronic and electronic solutions
have been employed with a great deal of success.
Though it is not possible to exactly separate the two halves of the
DHF solutions for molecular wavefunctions, methods such as the
Foldy-Wouthoysen transformation and the related Douglas-Kroll transformation
can produce a molecular Hamiltonian which is decoupled to a certain order in
the fine structure constant, 
. The resultant
Hamiltonians, and their two component solutions, restore the traditional,
familiar picture of molecular electronic structure
where all interactions are solely among electrons and nuclei.
This return to the two component picture also brings the advantage of
dramatically increased affordability of correlated methods.