Hi, I would like to know if there is a way to build a surface representation from a pdb file of a dummy atom saxs model in chimera. The file consist only of CA atoms. I could simulate this by increasing the vdw radius but the model becomes way to big. Thanks James

Hi James, You can use Multiscale Models to make a contour surface that encloses the "atoms." The tricky part may be adjusting the contour surface parameters to get the desired smoothness and enclosed total volume. At least in my tests, Chimera did not like the columns in your file after the B-factor column, so I made a version with those removed and opened it with "File... Open" before proceeding. I will send you that file in a separate message. (1) for purposes of comparison to the surface, I displayed the atoms as spheres with the radius 3.75 (this value was given in comments in the file) using the following commands: setattr a radius 3.75 rep sphere You can show the command line with Favorites... Command Line. (2) started Multiscale Models (under Tools... Higher-Order Structure), clicked the "Make models" button near the bottom. This generates the contour surface but hides the spheres. To also show the spheres, Ctrl-click on the surface to select it, and then in the "Style" line of the dialog, choose "Show also.. Spheres". To see spheres inside the surface, make it transparent; set Transparency to some fraction, say 0.3. (3) Then you can fiddle with the contour surface parameters in the Multiscale Models dialog. Many are hidden but can be shown by checking the box next to "..." I got a fairly decent enclosure of the spheres with all default parameters except setting the "Threshold atom density" for "CA only" to 0.0006 instead of the default 0.002. You could also fiddle with the smoothing parameters and resolution if that is not good enough. If you have some idea of what total volume the surface should enclose, that can be evaluated with Measure Volume and Area (under Tools... Volume Data). I hope this helps, 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