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 Huckel method), approximate evaluation of certain molecular integrals (see, for example,
Zero differential overlap approximation), simplification of the wavefunction (for example, use of a Pi (π) electron approximation
as in
Pariser–Parr–Pople method),
etc.
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