# [Chimera-users] Question about Measure Volume and Area

Elaine Meng meng at cgl.ucsf.edu
Thu Aug 14 20:42:39 PDT 2008

```Hi Patrick,
I'm having trouble getting a surface on that structure in the first
place, but setting that issue aside...

I guess you are using "Measure Volume and Area" since "Area/Volume
from Web" does not have an option for probe radius 2.0.  As discussed
on the "Measure Volume and Area" man page,

<http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/measurevol/
measurevol.html>

this tool calculates the volume and area directly from the
triangulated surface, not analytically.  However, the surface area
values reported in the Reply Log as soon as you show a molecular
surface are analytical, so use those, assuming you are able to
successfully generate a surface.

I can think of a couple possible explanations for nonintuitive results:
(a) there is more than one surface component (disconnected blob) and
"Measure Volume and Area" includes the total of both.  Maybe with one
probe size it also calculates the inside surface, with the other it
does not.  To make sure you are only measuring one component, use
<http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/pickblobs/
pickblobs.html>
(b) the surface calculation failed (maybe high symmetry makes this
more likely?) and the triangulation was incomplete or artifactual.
Seems like this would be visible, however.

Voronoi volume is a totally different thing than surface-enclosed
volume.  Each atom is partitioned into a cell, essentially, and the
surface atoms have infinite Voronoi volumes because there are no
neighbors in the outside direction to bound their cells.  That being
said, you can use "Area/Volume from Web" to calculate atomic Voronoi
volumes. I think it reports -1 for the infinite volumes. You might
also want to try calculating surface areas with that tool.  There are
a few different allowed probe radii, but 2.0 is not one of the choices.

<http://www.cgl.ucsf.edu/chimera/docs/ContributedSoftware/surfvol/
surfvol.html>

I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                          meng at cgl.ucsf.edu
UCSF Computer Graphics Lab (Chimera team) and Babbitt Lab
Department of Pharmaceutical Chemistry
University of California, San Francisco
http://www.cgl.ucsf.edu/home/meng/index.html

```