Austin Model 1, or AM1, is a semi-empirical method for the quantum calculation of molecular electronic structure in computational chemistry. It is based on the Neglect of Differential Diatomic Overlap integral approximation. Specifically, it is a generalization of the modified neglect of differential diatomic overlap approximation. Related methods are PM3 and the older MINDO.

AM1 was developed by Michael Dewar and co-workers and published in 1985. AM1 is an attempt to improve the MNDO model by reducing the repulsion of atoms at close separation distances. The atomic core-atomic core terms in the MNDO equations were modified through the addition of off-center attractive and repulsive Gaussian functions.

The complexity of the parameterization problem increased in AM1 as the number of parameters per atom increased from 7 in MNDO to 13-16 per atom in AM1.

The results of AM1 calculations are sometimes used as the starting points for parameterizations of forcefields in molecular modelling.

AM1 is implemented in the MOPAC, AMPAC, GAUSSIAN, CP2K, GAMESS (US), PC GAMESS, GAMESS (UK), and SPARTAN programs.

An extension of AM1 is SemiChem Austin Model 1 (SAM1), which is implemented in the AMPAC program and which explicitly treats d-orbitals.

A model for the AM1 calculation of lanthanide complexes, called Sparkle/AM1, was also introduced and is implemented in MOPAC2007.

AM1 has been recently reparameterized, leading to the new RM1, or Recife Model 1, available in MOPAC2007, SPARTAN software, Hyperchem 8, etc.

An extension of AM1 is AM1* that is available in VAMP software.

See also[link]

• Semi-empirical quantum chemistry methods

References[link]

• Dewar, Michael J. S.; Zoebisch, Eve G.; Healy, Eamonn F.; Stewart, James J. P. (1985). "Development and use of quantum mechanical molecular models. 76. AM1: A new general purpose quantum mechanical molecular model". Journal of the American Chemical Society 107 (13): 3902. DOI:10.1021/ja00299a024. 
• Dewar, Michael J.S.; Jie, Caoxian; Yu, Jianguo (1993). "SAM1; the first of a new series of general purpose quantum mechanical molecular models". Tetrahedron 49 (23): 5003. DOI:10.1016/S0040-4020(01)81868-8. 
• Freire, RO; Rocha, GB; Simas, AM (2005). "Sparkle model for the calculation of lanthanide complexes: AM1 parameters for Eu(III), Gd(III), and Tb(III)". Inorganic Chemistry 44 (9): 3299–310. DOI:10.1021/ic048530+. PMID 15847440. 
• Rocha, GB; Freire, RO; Simas, AM; Stewart, JJ (2006). "RM1: A reparameterization of AM1 for H, C, N, O, P, S, F, Cl, Br, and I". Journal of Computational Chemistry 27 (10): 1101–11. DOI:10.1002/jcc.20425. PMID 16691568. 
• Leach, Andrew R. (2001). Molecular Modelling. Pearson Education Limited. ISBN 0-582-38210-6. 

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