The coefficients C indicate the contribution of the determinant to the final wave function expansion and S, D, . . ., indicate the excitation level out of the reference state. The reference |R may be a single Slater determinant, such as a Hartree-Fock state, or it may be a CASSCF reference. In contrast to a linear expansion out of the reference that is truncated at some excitation level (e.g. configuration interaction with singles and doubles, CISD), the goal of sCI methods is to minimize the expectation value of the molecular electronic Hamiltonian Ĥ spanned by a subset of all possible determinants, without regard to an a priori truncation.