[Pw_forum] Ecutrho's choice

[Jess Kondor]

 Hi all,

  I have some doubts with choice of ecutrho. For my test material, I use
ultrasoft pseudopotentials, LDA. Using high ecutrho=400, I found that
Ecutwfc=40 Ry, 4x4x4 k-point mesh is enough in order to achieve convergence.
Then, I fixed these parameters and changed ecutrho. I found, that properties
of my test system [rhombohedral unit cell] (total energy, lattice constant,
angle between cell vectors, unit-cell volume) are almost identical to one
obtained by using ecutrho=160 Ry. :

===================================================================================================
 EcutRho (Ry)    Etot (Ry)                 alat (Ang)
Angle                           volume (ang**3)
===================================================================================================
   160       -983.62434066       5.47150468539004      60.1841848177694
116.30927040909165860688
   200       -983.62458218       5.47150604640841      60.1841524430312
116.30927040909165860688
   240       -983.62458762       5.47150640562892      60.1841431281284
116.30927040909165860688
   280       -983.62458015       5.47150677576742      60.1841338128593
116.30927040909165860688
   320       -983.62458383       5.47150716228288      60.1841244970402
116.30927040909165860688
   360       -983.62456511       5.47150753788046      60.1841151815896
116.30927040909165860688
   400       -983.62456736       5.47150792439603      60.1841058657727
116.30927040909165860688
   440       -983.62456667       5.47150829999372      60.1840965503242
116.30927040909165860688
   480       -983.62456522       5.47150868104283      60.1840871687159
116.30927040909165860688
   520       -983.62456799       5.47150905663303      60.1840777872924
116.30927040909165860688

I am a little bit confused, because in ultrasoft case ecutrho should be >
4*ecutwfc  . My system contains 3 atomic species, one of them is iron. I use
Fe.pz-sp-van_ak.UPF pseudopotential. I read the info about these
pseudopotentials (unfortunately, for GGA only). I saw that recommended value
of ecutho should be around 10-12*ecutwfc. What is the criteria for that
choice?

  Best regards,
    JK
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