SHMO Simple Hückel Molecular Orbital Calculator
SHMO Simple Hückel Molecular Orbital Calculator Version SHMo5 20110131
Orbital energies are displayed as "alpha + x beta"
(the coulomb integral, alpha, and the resonance integral, beta, are both negative)
Introduction to SHMO
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.
The first version of this program has been written entirely in the Java language by Richard Cannings. Some bug fixes and new options, and new display of the data and the molecular orbitals has been programmed by Hans-Ulrich Wagner.
Orbitals are displayed as red/green or lightgrey/grey coloured circles. Empty oribtals can be displayed with white centers, half filled with yellow centers. This option can be switched using the button "Occup".
Orbital energies are displayed as "alpha + x beta" (the coulomb integral, alpha, and the resonance integral, beta, are both negative).
Orbital selection is by clicking on the orbital energy display or step up/step down buttons.
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:
- All parameters used are exactly listed in the Data-Table.
- All parameters are now displayed using the on/off-option "Parameters".
- The on/off-option "Verbose" switches the display of orbital coefficients while showing orbitals.
- The "On-the-fly Calculation" is active by default, even when changing heteroparamaters. On modern computers it must not be switched off.
- Atoms and bonds may be Added, Erased, or their types Changed at any time by simple mouse clicks. The number of electrons may be adjusted at any time and the calculated net charges and bond orders are displayed in the "Show Molecule" mode.
- All other buttons are self explanatory. Simply try it and analyse the result.
- All aspects of the calculation are printable by copying the data from [Show Data Table]. These tables and columns are optimized and show all relevant input and output in exact ASCII format. To get the Data-Table in an editor, put the mouse pointer down to the last line, use [Shift][arrow up] and mark the text up to the first line. Use [CTRL][C] to copy these lines into the clipboard and [CTRL][V] to paste it in an editor.