Logo kimball

The quantitative
G.N.Lewis Model

  Best of two worlds

  Kimball combined with R. Hoffmann's EHT

Kimballs model can compute a groundstate energy and an equilibrium structure of a molecule. Both are missing in Roald Hoffmann's original Extended Hückel program. The latter can compute wavefunctions, orbital energylevels and LCAO-MO components, neglecting electron repulsion. In the magic hands of Roald Hoffmann, EHT has been one of the major contributors to the understanding of chemistry during 50 years! Just look at and read any of hundreds of Hoffmann's papers in JACS to get an impression of how his careful arguments deal with and solve simple and difficult chemical problems. He is usually based on EHT computations, but never dominated by them. Sometimes he includes first principles QC, when his simple program fails to give an unambiguous answer.I like EHT and have been using it for decades, especially in the varieties Iconc and Icon-Edit which Gion Calzaferri and his coworkers have developed. They successfully added a reasonable extension to make EHT sensitive to electron repulsion.

This has motivated me to combine Kimball and EHT: Perego's Kimball.exe determines a groundstate energy and a structure. After a successful Kimball run, an option writes an input file for EHT using Kimball's structure. This is automatically opened and run with the EHT program. Finally, the output file is written and opened in an editor, from where it may be studied and printed.

If you are interested to look at this, download Kimbicon.zip, unpack it into any directory on your Windows machine. Open a DOS window and write "kimbicon 'your molecule'", where 'your molecule' is the name of what you want to compute. Now Kimball.exe opens. Compose your molecule, optimize the structure. When converged, save the run as EHT file, quit Kimball.exe. EHT begins to run automatically and soon the results are presented in the editor. Currently, the older version Iconc-1997 of Calzaferri's EHT and a PDF manual are in the zip.-There is one slight problem connected to the old Modula-2 programming language of Kimball.exe: You must be using a 32-bit Windows version XP or older! The EHT program needs a Windows environment, hence you cannot use DOSbox (good for Kimball.exe, but EHT does not run). The batch file and both programs run on the virtual WindowsXP which Windows7 offers for free. If the batchfile cannot be used in your environment, just make it a two step process: Run Kimball.exe in DOSbox and output the .ico file, then exit. Now run Iconc under Windows with: Iconc < 'molecule'.ico > 'molecule'.out

Just one big Caveat: Hoffmann's EHT does NOT explain chemical bonding! That is not its intention! Without electron-electron repulsion (and, of course, nuclear repulsion and electron-nuclear attraction), no chemical bond exists.