Semi-empirical
quantum chemistry methods are based on the
Hartree–Fock formalism, but make many approximations and obtain some parameters from empirical data. They are very important in
computational chemistry for treating large molecules where the full Hartree–Fock method without the approximations is too expensive. The use of empirical parameters appears to allow some inclusion of
electron correlation effects into the methods.
Within the framework of Hartree–Fock calculations, some pieces of information (such as two-electron integrals) are sometimes approximated or completely omitted. In order to correct for this loss, semi-empirical methods are parametrized, that is their results are fitted by a set of parameters, normally in such a way as to produce results that best agree with experimental data, but sometimes to agree with
ab initio results.
Semi-empirical calculations are much faster than their ab initio counterparts, mostly due to the use of the
zero differential overlap approximation. Their results, however, can be very wrong if the molecule being computed is not similar enough to the molecules in the database used to parametrize the method.
Preferred application domains
Methods restricted to π-electrons
These methods exist for the calculation of electronically excited states of polyenes, both cyclic and linear. These methods, such as the
Pariser–Parr–Pople method (PPP), can provide good estimates of the π-electronic excited states, when parameterized well.[8][9][10] For many years, the PPP method outperformed ab initio excited state calculations.
Methods restricted to all valence electrons.
These methods can be grouped into several groups:
Methods such as
CNDO/2,
INDO and
NDDO that were introduced by
John Pople.[11][12][13] The implementations aimed to fit, not experiment, but ab initio minimum basis set results. These methods are now rarely used but the methodology is often the basis of later methods.
Methods that are in the
MOPAC,
AMPAC,
SPARTAN and/or
CP2K computer programs originally from the group of
Michael Dewar.[14] These are
MINDO,
MNDO,[15]AM1,[16]PM3,[17] PM6,[18] PM7[19] and
SAM1. Here the objective is to use parameters to fit experimental heats of formation, dipole moments, ionization potentials, and geometries. This is by far the largest group of semiempirical methods.
Methods whose primary aim is to calculate excited states and hence predict electronic spectra. These include
ZINDO and
SINDO.[20][21] The OMx (x=1,2,3) methods[22] can also be viewed as belonging to this class, although they are also suitable for ground-state applications; in particular, the combination of OM2 and
MRCI[23] is an important tool for excited state molecular dynamics.
Tight-binding methods, e.g. a large family of methods known as
DFTB,[24] are sometimes classified as semiempirical methods as well. More recent examples include the semiempirical quantum mechanical methods GFNn-xTB (n=0,1,2), which are particularly suited for the geometry, vibrational frequencies, and non-covalent interactions of large molecules.[25]
The NOTCH method[26] includes many new, physically-motivated terms compared to the NDDO family of methods, is much less empirical than the other semi-empirical methods (almost all of its parameters are determined non-empirically), provides robust accuracy for bonds between uncommon element combinations, and is applicable to ground and excited states.
^Hückel, Erich (1932). "Quantentheoretische Beiträge zum Problem der aromatischen und ungesättigten Verbindungen. III". Zeitschrift für Physik (in German). 76 (9–10). Springer Science and Business Media LLC: 628–648.
Bibcode:
1932ZPhy...76..628H.
doi:
10.1007/bf01341936.
ISSN1434-6001.
S2CID121787219.
^Pariser, Rudolph; Parr, Robert G. (1953). "A Semi‐Empirical Theory of the Electronic Spectra and Electronic Structure of Complex Unsaturated Molecules. I.". The Journal of Chemical Physics. 21 (3). AIP Publishing: 466–471.
Bibcode:
1953JChPh..21..466P.
doi:
10.1063/1.1698929.
ISSN0021-9606.
^Pariser, Rudolph; Parr, Robert G. (1953). "A Semi‐Empirical Theory of the Electronic Spectra and Electronic Structure of Complex Unsaturated Molecules. II". The Journal of Chemical Physics. 21 (5). AIP Publishing: 767–776.
Bibcode:
1953JChPh..21..767P.
doi:
10.1063/1.1699030.
ISSN0021-9606.
^Pople, J. A. (1953). "Electron interaction in unsaturated hydrocarbons". Transactions of the Faraday Society. 49. Royal Society of Chemistry (RSC): 1375.
doi:
10.1039/tf9534901375.
ISSN0014-7672.
^J. Pople and D. Beveridge, Approximate Molecular Orbital Theory, McGraw–Hill, 1970.
^Michael J. S. Dewar & Walter Thiel (1977). "Ground states of molecules. 38. The MNDO method. Approximations and parameters". Journal of the American Chemical Society. 99 (15): 4899–4907.
doi:
10.1021/ja00457a004.
^Michael J. S. Dewar; Eve G. Zoebisch; Eamonn F. Healy; James J. P. Stewart (1985). "Development and use of quantum molecular models. 75. Comparative tests of theoretical procedures for studying chemical reactions". Journal of the American Chemical Society. 107 (13): 3902–3909.
doi:
10.1021/ja00299a024.
^James J. P. Stewart (1989). "Optimization of parameters for semiempirical methods I. Method". The Journal of Computational Chemistry. 10 (2): 209–220.
doi:
10.1002/jcc.540100208.
S2CID36907984.
^Nanda, D. N.; Jug, Karl (1980). "SINDO1. A semiempirical SCF MO method for molecular binding energy and geometry I. Approximations and parametrization". Theoretica Chimica Acta. 57 (2). Springer Science and Business Media LLC: 95–106.
doi:
10.1007/bf00574898.
ISSN0040-5744.
S2CID98468383.