Editing Computational chemistry (section)

=== Semi-empirical methods ===
{{Main article|Semi-empirical quantum chemistry methods}}

Semi-empirical [[quantum chemistry]] methods are based on the [[Hartree–Fock method]] formalism, but make many approximations and obtain some parameters from empirical data. They were very important in computational chemistry from the 60s to the 90s, especially for treating large molecules where the full Hartree–Fock method without the approximations were too costly. The use of empirical parameters appears to allow some inclusion of correlation effects into the methods.<ref name="Ramachandran-2008">{{Cite book |last1=Ramachandran |first1=K. I. |title=Computational chemistry and molecular modeling: principles and applications |last2=Deepa |first2=G. |last3=Namboori |first3=K. |date=2008 |publisher=Springer |isbn=978-3-540-77304-7 |location=Berlin}}</ref>

Primitive semi-empirical methods were designed even before, where the two-electron part of the [[Hamiltonian (quantum mechanics)|Hamiltonian]] is not explicitly included. For π-electron systems, this was the [[Hückel method]] proposed by [[Erich Hückel]], and for all valence electron systems, the [[extended Hückel method]] proposed by [[Roald Hoffmann]]. Sometimes, Hückel methods are referred to as "completely empirical" because they do not derive from a Hamiltonian.<ref>{{Cite journal|last=Counts|first=Richard W.|date=1987-07-01|title=Strategies I|journal=Journal of Computer-Aided Molecular Design|language=en|volume=1|issue=2|pages=177–178|doi=10.1007/bf01676961|pmid=3504968|issn=0920-654X|bibcode=1987JCAMD...1..177C|s2cid=40429116}}</ref> Yet, the term "empirical methods", or "empirical force fields" is usually used to describe molecular mechanics.<ref>{{Cite book|title=Reviews in Computational Chemistry|last1=Dinur|first1=Uri|last2=Hagler|first2=Arnold T.|date=1991|publisher=John Wiley & Sons, Inc.|isbn=978-0-470-12579-3|editor-last=Lipkowitz|editor-first=Kenny B.|pages=99–164|language=en|doi=10.1002/9780470125793.ch4|editor-last2=Boyd|editor-first2=Donald B.}}</ref>

[[File:MM_PEF_3.png|thumb|Molecular mechanics potential energy function with continuum solvent]]