RNA molecules can be designed by applying an optimization method (adaptive walk, Monte-Carlo, genetic algorithm, constraint programming, etc) to compute sequences that are likely to fold into a given target structure. Since suboptimal structures can be quite distinct from a desired target structure, it is likely to be more important to minimize the structural diversity around a target structure, than to ensure that the minimum free energy structure is equal to the target structure. A number of measures of structural diversity have been introduced in the literature, ranging from positional entropy to ensemble defect. Here, we describe our C-program that efficiently computes such measures; in the future, source code will be made available at our web site http://bioinformatics.bc.edu/clotelab/.