[Pw_forum] Ecutrho's choice

[Paolo Giannozzi]

On Thursday 25 May 2006 20:59, Jess Kondor wrote:

> 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.
> [...] I am a little bit confused, because in ultrasoft case ecutrho should
> be > 4*ecutwfc  .

it may be, but not necessarily so. If it isn't, good for you.
Consider that 40 Ry is a relatively high cutoff for Ultrasoft PP's. 
One tries to perform calculations with ecutwfc = 25 Ry or so.

> 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.

GGA PP's are typically nastier than LDA ones.

Paolo
-- 
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
Piazza dei Cavalieri 7      I-56126 Pisa, Italy