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The article demonstrates rather general approach to problems of discretegeometry: treat them as global optimization problems to be solved by one ofgeneral purpose solver implementing branch-and-bound algorithm (B&B). Thisapproach may be used for various types of problems, i.e. Tammes problems,Thomson problems, search of minimal potential energy of micro-clusters, etc.Here we consider a problem of densest packing of equal circles in specialgeometrical object, so called square flat torus $\mathbb{R}^2/\mathbb{Z}^2$with the induced metric. It is formulated as Mixed-Integer Nonlinear Problemwith linear and non-convex quadratic constraints.  The open-source B&B-solver SCIP, http://scip.zib.de, and its parallelimplementation ParaSCIP, http://ug.zib.de, had been used in computingexperiments to find "very good" approximations of optimal arrangements. Themain result is a confirmation of the conjecture on optimal packing for N=9 thatwas published in 2012 by O. Musin and A. Nikitenko. To do that, ParaSCIP tookabout 2000 CPU*hours (16 hours x 128 CPUs) of cluster HPC4/HPC5, NationalResearch Centre "Kurchatov Institute", http://ckp.nrcki.ru

Packing of Circles on Square Flat Torus as Global Optimization of Mixed Integer Nonlinear problem