Hi Dieter, Just a few add-ons to Tom's reply: Another approach to getting the surface area of the receptor buried by the ligand is to run Surface Area/Volume (a) on the complex of ligand and receptor (must delete water and anything else first). Then use Attribute Calculator to sum the surface areas of the receptor atoms only. (b) after deleting ligand, run it again on the receptor. The total is all from the receptor, so it is not necessary to use Attribute Calculator. Subtract the value from (a) from this value. Tom gave a link to the development documentation, which is slightly ahead of the Chimera version you have. It is almost the same, except that the Tools menu has been reorganized. Probably in the version you have, both Attribute Calculator and Surface Area/Volume are in the Utilities submenu of the Tools menu. To get the documentation that goes with your version, use the Help buttons on those dialogs or open the User's Guide from the Help menu. If you have the latest snapshot (1.2105, March) there is an "Attributes" tutorial (Help... Tutorials). Part 2 has one example of using Surface Area/Volume and Attribute Calculator. Finally, if anything is confusing or hard to find in the documentation, please feel free to complain to me! 8-) Elaine ----- Elaine C. Meng, Ph.D. meng@cgl.ucsf.edu UCSF Computer Graphics Lab and Babbitt Lab Department of Pharmaceutical Chemistry University of California, San Francisco http://www.cgl.ucsf.edu/home/meng/index.html