[Pw_forum] question on ecutwfc and ecutrho

[Xunlei Ding]

Dear all,

I am now doing a calculation on a adsorption system.
To determine the ecutwfc and ecutrho,
I calculate the scf energies of a (3*2*5) slab with two adsorbed CO2.

unit:ryd
ecutwfc    ecutrho    total energy
24           288      -2725.94125538              case1
32           140      -2726.54980713              case2
32           200      -2726.06221809              case3
35           288      -2726.04364368              case4

Question1: It is said that the ecutrho should be about 4*ecutwfc, but
why the difference between case2 and case3 is so large?

Question2: It the difference between case3 and case4 small? (The
adsorption energy is less than 0.5eV. )

Question3: Can I determine the suitable values for ecutwfc and ecutrho
from these data?

Thank you!

Yours, 
Ding