·
Ab initio calculations require a lot of computer time and
memory. This is because many difficult
integrals involving the atomic orbitals have to be calculated for the electron-electron
interactions.
·
So-called ‘semiempirical methods’ are much faster, and can be
applied to bigger molecules (e.g. 100s of atoms instead of 10s of atoms).
·
In semiempirical methods, fewer integrals have to be calculated
because simplifying approximations are made.
·
The integrals that do remain are not calculated directly, but
instead are replaced by simple functions.
·
These simple functions are parameterized so that the semiempirical
method reproduces experimental data (e.g. molecular structures, dipole moments,
ionization energies, etc.) as well as possible. The experimental data contain the effects of electron
correlation, and so semiempirical methods can be a good choice in some
cases.
·
However, the approximations used make them often somewhat
inaccurate.
·
Also, they can only reproduce properties of molecules similar to
those used in parameterizing the method.
Usually only data for stable molecules are included, which means that
transition states may not be dealt with well.
·
Examples of semiempirical molecular orbital methods are MNDO, AM1 and PM3.