The Configuration Interaction (CI) method is often employed by chemists when studying atoms or small molecules. In this method is minimised by searching through all which are linear combinations of N-electron determinants made up from a total of M basis states (M>>N). This is obviously impractical to do thoroughly since there are
N-electron determinants and therefore the matrix to be diagonalised has dimension . Hence Configuration Interaction calculations usually only include determinants close to the Hartree-Fock ground state determinant. This means ignoring all high energy states, so the accuracy of these Configuration Interaction calculations is not as good as results found using Quantum Monte Carlo. Configuration Interaction calculations are not possible in solids due to the large number of determinants which would be required, however they can provide highly accurate results for small systems. In other words, CI calculations scale very badly with system size.