Hello,

it's quite unusual that define is not able to construct useful redundant internal coordinated with the 'ired' option. But it can happen for molecules with a very complicated structure (several rings and cages, possibly mixed with intermolecular connections are a nightmare). Than that easiest way out is to do the geometry optimization in cartesian coordinates. This is not very efficient, but sometimes the only solution. After some geometry cycles in cartesian coordinates one can try again to construct redundant internal coordinates.

christof