Results of running plain Armijo bactracking

I disabled the quadratic branch in the linesearch, which forces backtracking by cutting alpha by a factor of 2. This new scheme performs much worse as I get failed linesearches in systems where the indenter has barely interacted with the lattice. Besides, there are failed linesearches on each step of the indenter :(

On inspecting the output from the linemin, it became clear that the failed linesearch arose due to |de_ideal| being less than IDEAL_TOL. I think that this condition is caused because we are very close to the minimum to start with. In this specific instance f.s didn't even change sign (it remained negative).
Note: All the above observations were made with IDEAL_TOL = 1e-10

Next I tried to minimize the first indentation configuration with IDEAL_TOL = 1e-18. With this new IDEAL_TOL the backtracker performed much better but I still got a failed linesearch. Despite f.s changing sign, the backtracker failed to identify the energy minimum because the Armijo sufficient decrease condition wasn't satisfied. Eventually, because of cutting alpha by a factor of 2, a failed linesearch arose when: |de_ideal| < IDEAL_TOL.

Conclusion: backtracker needs more muscle !