I just installed an eCatalogs, and now Chem3D does not work.
If you installed the eCatalog into the default location, and it installs over your existing Chem3D, this problem will occur. The problem is that the shared libraries between the eCatalogs application and Chem3D 4 are incompatible. The solution is to install the eCatalogs into a separate location. If you install them into separate locations (using the Select Folder option at the bottom of the Install dialog box), you should have no problems.