Backtrack line minimisation can fail

[davidbowler]

The backtracking line minimiser applies an Armijo criterion to decide whether a change in energy is acceptable. Under some circumstances this can lead to a failure to perform a line minimisation as the largest drop in energy does not satisfy the criterion.

(Unfortunately the only example I have available is for a large, five hundred atom system; it is attached for reference - the failure is in cell optimisation.)

ArmijoFail.zip