Hi, I am trying to get some rough estimates for the overall diameter and cavity size of the following 5 structures... 1c8d (diameter = ~200A from the literature) 1stm 1hqk 1jh5 (diameter = ~ 200A from the literature) 1c8n (diameter = ~ 280A from the literature) Specifically 1stm and 1hqk, for which I can't find an estimate in the literature (admittedly only after a quick look). I decided to tried using Rasmol, starting with 1c8d. I quickly found that I couldn't get the structures from the PQS, which showed the correct structure (60-mer) but the link to the file was broken. Conversely, the pdb-beta site appeared not to know about the correct biological assembly (showing only the ASU), but gave me the correct (60 subunit file) anyway. Using the 'pick distance' feature in Rasmol proved to be a bit buggy, sometimes returning a 'nan' distance at the terminal (apparently labeled correctly on the structure) but worse, sometimes returning a distance that seemed far too small (giving values in the range of 30 to 60 A instead of 100 to 200 A by eye). Computationally what I would like to do is fit a sphere (or an ellipse, whichever is smaller) to the structure and minimize the size of the sphere to enclose something like 95-99% of the atoms. Is there any tool that I can use to do this? I am not even sure if this description makes sense, but I want to avoid just returning the maximum distance between any two atoms, which probably isn't a fair 'rough estimate' of the diameter of the above molecules. Does anyone happen to know (approximately) the external and internal diameter of these molecules? Or a (free) package that lets me find them? Thanks for any suggestions, Dan.