Hi Yasser,
if you want to generally select all singleton atoms (atoms not bonded to other atoms), you could use this command:

sel @/!bonds

this is an example of specification by attribute, described here:
<http://www.cgl.ucsf.edu/chimera/docs/UsersGuide/midas/atom_spec.html#descriptors>

I hope this helps,
Elaine
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Elaine C. Meng, Ph.D. 
UCSF Computer Graphics Lab (Chimera team) and Babbitt Lab
Department of Pharmaceutical Chemistry
University of California, San Francisco

On Aug 10, 2010, at 7:49 AM, Yasser Almeida Hernández wrote:

> Hi...
> The problem is that I have hundreds of pdb files with this format: four models (or less), where each one is an aminoacid and a single atom. Mostly, the single atom is an heteroatom and i select it with :/isHet, but sometimes this atom is a protein atom with the ATOM field and then, the :/isHet selection doesn't work...
> There is some tricky way to select this single atom types (ATOM), according to the structure of the pdb file...???
> Thanks

.