Hello Siyoung,
Sorry no, I can't think of a way to do it with a ChimeraX command. Maybe you could do it by using the initial orientation of the structure (just opening and not rotating) and veerrryyy carefully using the mouse to select a rectangle (Ctrl-click drag) but that is probably too difficult.  Also you would need to know where X=0 is from just looking at the structure.

Certainly you could do it with Python, but somebody else would have to advise on that.

Best,
Elaine
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Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Oct 17, 2023, at 5:33 AM, Siyoung Kim via ChimeraX-users <chimerax-users@cgl.ucsf.edu> wrote:
>
> Hello,
>
> Would there be a selection keyword that selects atoms based on their coordinates? For instance, is there a way that I can select atoms whose x coordinates are below 0 (e.g., x < 0)?
>
> Thank you.
> Best,
> Siyoung