Hi Elham, Testing Pseudopotentials and choosing optimal ones is a daunting task that needs some serious work. In addition to the valuable advice you quoted from Prof. de Gironcoli, I'd add that converging total energy will lead you to a very safe e_cut and rho_cut but probably one can afford a little less accuracy than the very safe one. The way to test is to focus on the quantity you care about and try to check its convergence.
See for example this descent work on testing PP's and finding optimal cutoffs based on phonon frequency convergence and equation of state error with respect to all electron calculations (among other things): http://materialscloud.org/sssp/ A similar example is GBRV library for which the criteria for convergence (as stated in the authors paper) is energy difference rather than absolute total energy: http://www.physics.rutgers.edu/gbrv/ I hope this helps. M. Y. MIT
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