Hello,

I am trying to place a centroid marker at the center of a selection of atoms, for example in the middle of a benzene ring that is at some arbitrary position in my system. I want to do some distance measurements here. I can do something like this:

shape sphere color orange radius 0.25 center sel

where the center sel places the sphere where I want, but these spheres can't be used to make measurements. Instead I think I need a marker object, such as:

define centroid radius 0.25 color orange

But I don't know how to recreate that `center sel` position. I have tried 

define centroid radius 0.25 color orange position center
 
define centroid radius 0.25 color orange position center sel

define centroid radius 0.25 color orange position sel

These do not work. I am under the impression that the centroid is a marker object and should have some position argument. Can anyone help me?

Thanks,

--
German Barcenas
Ph.D Candidate | Materials Science and Engineering 
Materials Modeling and Theory Group
Boise State University