Hi Geoff, If the main reason you want to write multiple PDBs to a file is to preserve their relative orientations, you can essentially do that with "write PDB" successive times on different models but always relative to the same model. I concede that could be tedious, however. Pdbrun cat is a little dicey since you need to make sure all the atoms you care about are displayed, since only those will be included. Also, with multiple models, I think it puts an END record in between, which needs to be removed if you are going to reopen the file and display the models. As you probably noticed, a lot of USER records get included in the file unless you use the keyword nouser. Hope this helps, Elaine =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Elaine C. Meng, Ph.D. Babbitt Lab and Computer Graphics Lab meng@cgl.ucsf.edu http://www.cgl.ucsf.edu/home/meng/index.html =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=