This information is in the Help page for this tool. 

You can click the Help button on the "Minimize Structure" dialog or see the copy on our website here:
<https://www.rbvi.ucsf.edu/chimera/docs/ContributedSoftware/minimize/minimize.html>

Specifically, see the section "Force Field Parameters" and "nonstandard residues"
<https://www.rbvi.ucsf.edu/chimera/docs/ContributedSoftware/minimize/minimize.html#nonstandard>

Short answer is that is uses GAFF (see details in link above) with the type of charges you chose in the dialog, from your image Gasteiger.

I hope this helps,
Elaine
-----
Elaine C. Meng, Ph.D.                       
UCSF Chimera(X) team
Resource for Biocomputing, Visualization, and Informatics
Department of Pharmaceutical Chemistry
University of California, San Francisco

> On Jan 13, 2024, at 1:33 AM, Prabuddha Bhattacharya via Chimera-users <chimera-users@cgl.ucsf.edu> wrote:
>
> Dear Sir
>
> I would be grateful if you kindly let me know what force field is used by Chimera to optimize the geometry of a small organic molecule (not peptide) if I minimize the structure using the parameters as shared in the attached file.
>
> Thank you
>
> Kind regards
>
> Prabuddha Bhattacharya