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SHMO (Read 4978 times)
Gerrit-Jan Linker
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Posts: 75
02.01.13 at 09:53:23
SHMO stands for Simple Huckel Molecular Orbital program.  
SHMO is an interactive program to perform electronic structure calculations within the "Simple Hückel Molecular Orbital" approximations.  
The theoretical basis for the method is described in "Orbital Interaction Theory of Organic Chemistry", Second Edition by Arvi Rauk (Wiley Interscience, 2001), Chapters 3 and 5.
SHMO is designed as a teaching aid and replaces the earlier version distributed with the book. Besides performing conventional SHMO calculations, SHMO permits easy changes of orbital electronegativities (the Hückel coulomb integrals, alpha, hX) and intrinsic orbital interaction values (the Hückel resonance integrals, beta, kXY) to illustrate the effects of interacting orbital energies orbital, energy differences and overlaps on the resultant molecular orbital energies and polarizations.
Orbital energies are displayed as "alpha + x beta" (the coulomb integral, alpha, and the resonance integral, beta, are both negative).
Heteroatoms, X, are incorporated as aX = alpha + hX beta, and bXY = kXY beta , where the default values of the parameters, hX and kXY, are taken from the publication of F. A. Van-Catledge, J. Org. Chem. 1980, 45, 4801-4802.
Thus for C=C, hC = 0.0 and kCC = 1.0; f.e. for C=O hC=0.0, hO=0.97 and kCO=1.06.
Obs.: Changing the hX-parameters for atoms changes also immediately the kXY-bond-parameters for the bonds to the neighbors. These kXY-bond-parameters have to be changed after the hX-atom-parameters.
The naming of the heteroatoms has been changed and corresponds now to the publication of Van-Catledge cited before. The number of the contributed pi-electrons is given. N2e means an amino-group having 2 pi-electrons f.e. as in aniline or pyrrol. N1e is an aza-atomcenter having one pi-electron f.e. as in pyridine. A ketone has a O1e-atom and furan has a O2e-atom. For atoms with an obvious number of electrons no number is named.
NOTE: This above description corresponds to the standard definition in SHMO but differs from the convention adopted in "Orbital Interaction Theory of Organic Chemistry", Second Edition by Arvi Rauk. A version of SHMO that is compatible with the book may be found at:  
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Gerrit-Jan Linker
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