Thanks Elaine, but after I combine using the syntax that you mentioned, I get the entire molecule and not the chains that I selected. 

I also tried selecting the that chains that I want to combine from different models, and used "combine sel", I got the same result. 

On Wed, Jan 30, 2019 at 6:21 PM Elaine Meng <meng@cgl.ucsf.edu> wrote:
Hi Ahmad,
Commas separate items at the main symbol level # (model) : (residue) @ (atom).  They cannot be used to separate items at the sub-levels with dots after those symbols   #. (submodel)  :. (chain ID)  @.  (atom alternate location).

<http://www.rbvi.ucsf.edu/chimera/docs/UsersGuide/midas/atom_spec.html#hierarchy>

I agree it can be confusing.  In our next-gen program ChimeraX we only use the dot for submodels, and chain has its own symbol.
Best,
Elaine

> On Jan 30, 2019, at 2:18 PM, Ahmad Khalifa <underoath006@gmail.com> wrote:
>
> Thanks, I will try that, but quickly, I often use commas with selection and it works fine, what's the difference between the commas and the colons?
>
> On Wed, Jan 30, 2019 at 4:03 PM Elaine Meng <meng@cgl.ucsf.edu> wrote:
> Hi Ahmad,
> The specification is wrong for two reasons:
> (1) need colon instead of comma, for example #1:.k:.e
> (2) you would need to either remove the space between the two models or put quotes around the whole thing like "#1:.k:.e #2:.k:.e"
>
> However, more important is that “combine” will include all the atoms in the whole models even if you only specify some chains. It says this in the second paragraph:
> <http://www.rbvi.ucsf.edu/chimera/docs/UsersGuide/midas/combine.html>
>
> So what you really wanted to do is to delete all the other atoms before you combine the two models, something like
>
> select #1:.k:.e #2:.k:.e
> delete ~sel
> combine #1,2
>
> (also make sure you really have models #1 and #2, since the first model is #0 and you might need to use #0 and #1 instead… check in the Model Panel, opened from Favorites menu)
>
> I hope this helps,
> Elaine
> -----
> Elaine C. Meng, Ph.D.
> UCSF Chimera(X) team
> Department of Pharmaceutical Chemistry
> University of California, San Francisco
>
> > On Jan 30, 2019, at 11:23 AM, Ahmad Khalifa <underoath006@gmail.com> wrote:
> >
> > I'm trying to combine a few chains from different models. I'm using: combine #1:.k,.e #2:.k,.e. I'm getting the above error, and I'm not sure why my selection isn't working!
> >
> > Regards. 
>
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