One of the most conceptually straightforward methods of obtaining a better wavefunctions is given by the configuration interaction (CI) method. The CI method employs a wavefunction which is constructed of a linear combination of the DHF wavefunction with excited determinants
The expansion coefficients are then selected so that they variationally minimize the expectation value of the electronic energy with respect to the CI wavefunction
where the wavefunction is restricted by the normalization condition
In the most common implementation of this method, the wavefunction is constructed only from certain classes of excited determinants. The methods used to select the best set of excited determinants, including a great deal of recent work in the area of restricted active space (RAS) selection schemes[10], have been the subject of much discussion. The wavefunction which has been constructed from all the possible excited determinants is known as the full CI (FCI) wavefunction, and represents the exact solution of the DC Hamiltonian within the space constructed of the electronic DHF spinors. An efficient version of a DHF based RAS CI program has been implemented by Visscher and coworkers in the MOLFDIR[11] program package.