YE OLDE EXTENDED HUCKEL CODES
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The first program is CACAO short for Computer Aided Composition of Atomic Orbitals and it was made a long time ago by Carlo Mealli and Davide Proserpio. In order to run it you must disable your graphics card in Windows 7 or later and use the default Windows standard VGA graphics card (no Aero or glassy effects). It has full use of symmetry and carries a molecular symmetrization routine now migrated to ADFinput. No extended systems' capability other than using a CCSD .fdat format to import geometries.
http://www.chembio.uoguelph.ca/oakley/310/cacao/cacao.htm
Of the extended structure codes the first is my favourite because it's easier to use. It's Greg Landrum's Yet Another Extended Huckel Package (YAEHMOP) and naturally has its uses in the molecular world but no symmetry analysis is possible.
http://yaehmop.sourceforge.net/
Also a useful and visual program is the CAESAR ensemble of programs (Crystal And Electronic Structure AnalyzeR) by Myung-Hwan Whangbo.