By just copying self.back_calc_a to self.back_calc, problem was solved.
In specific_analysis.relax_disp.optimisation in function back_calc_r2eff(), the
function gets the last values stores in the class function.
This is in:
class Disp_result_command(Result_command):
"""Class for processing the dispersion optimisation results.
With:
self.back_calc = back_calc
And back_calc_r2eff() have return model.back_calc
Task #7807 (https://gna.org/task/index.php?7807): Speed-up of dispersion models for Clustered analysis.
