This option uses the molecular surface solver written by Amitabh Varshney when he was at the University of North Carolina. When this option is used, the radii and coordinates are written to a temporary file and the 'surf' executable is run with the probe radius as a parameter. When finished, the output is written to another temporary file which is then read by VMD and colored and displayed. The value of the probe radius is controlled by the sphere radius, and this is identical to the probe size in Å.
This surface is rather slow in both generation and display for systems over several hundred atoms. The SURF calculation is quite exact and will show complete detail even when it isn't needed. The use of disk space as a communications medium takes up about half the time, and the method used to draw the triangles has not been optimized.
There are two environment variables which can affect the Surf display option:
A helpful trick when constructing surfaces is to use the ``Apply Changes Automatically'' toggle button on the graphics form wisely. That is, since surfaces often take a long time to build, changing viewing parameters such as the probe radius can cause long delays. By default, each time you hit the probe radius button, VMD rebuilds the surface. If you want to reduce or enlarge the probe radius by several increments, then you would end up rebuilding the surface multiple times. By toggling the afore-mentioned button, you can force VMD to update on your command only. This trick is sometimes helpful with other representations as well.
For a faster though arguably more imprecise surface rendering method, see the description of MSMS below.