Search for the steady-state \((\ell,d)\) of a cell doublet for the given physical parameters (by minimizing total energy).
Steady-state (optimal) \((\ell_0,d_0)\) values.
tuple[float, float]
Note
\(\ell\) is the maximal cell radius, and \(2d\) is the cell-center distance of a doublet (rather than \(d\)).