i have a problem visualizing the cube files generated with gaussian'03. i'm using molekel_macos_5.2.0 and i can't use the Electron Density utility. i can get into the option from the surface menu but all the options from the Electron Density menu appear unabled. i have no problem using Grid Data but i would like to do as in the old version of molekel where i can add or subtract orbitals, use both signs.... can anybody help me? thanks!

The reason for which the Electron Density is not accessible is that no electron density data is available. Data in the .cube files can be anything e.g. electrostatic potential; all you have in .cube files is a 3d grid of float values, that's it. Regarding using +/- signs in the current version of Molekel, you can simply reload a second copy of the molecule and generate a new surface specifying a different isovalue. While this is definitely a workaround it's also quite handy because you can e.g. use one molecule to render all the surfaces with positive isovalues, another to render all the surfaces with negative isovalues and then toggle the visibility of each molecule to display only the surfaces with a specific isovalue.