Methods which use parameters derived from experimental data to simplify computations. The simplification may occur at various levels: simplification of the Hamiltonian (e.g., as in the Extended Hückel method), approximate evaluation of certain molecular integrals (see, for example, zero differential overlap (ZDO) approximation), simplification of the wavefunction (for example, use of a Pi (π) electron approximation as in Pariser–Parr–Pople method), etc.