Slater determinant
where formula_5 is the reference wave function, which is typically a Slater determinant constructed from Hartree–Fock molecular orbitals, though other wave functions such as configuration interaction, multi-configurational self-consistent field, or Brueckner orbitals can also be used. formula_6 is the cluster operator, which, when acting on formula_5, produces a linear combination of excited determinants from the reference wave function (see section below for greater detail).