[Pw_forum] Does PWSCF have a bug when optimizing the structure in one electric field?

[Giovanni Cantele]

xqhu at duke.edu wrote:
> Hi, Dear Users,
> I tried to optimize the molecular structure in a saw-like electric
> field using espresso-3.2. However, the optimization is always failed.
> I checked the forces through the numerical method. It seems that the
> forces in a electic field are wrong.
> Does anybody have the same problem?
>

I'm not sure if I remember well, but some time ago I tried to use the
utility for a sample system, a water molecule
in an orthorhombic supercell.
The point was that if the molecule were placed in the middle of the
supercell, weird result came out, in particular concerning the
total force. The problem seemed to disappear after centering the
molecule around (0,0,0). In this last case, the total force was
much more realistic.
Just to show you what happened, I used
ibrav=6
a=3.0
b=3.0
c=10.0
and
edir=3
eamp=0.D0
eopreg=0.1
in both runs. But with emaxpos=0.9 and
ATOMIC_POSITIONS { Angstrom }
O 1.5 1.5 4.69
H 2.268 1.5 5.31
H 0.732 1.5 5.31
the result was: ! total energy = 235.03525091 ryd Total force = 17.072620
whereas, with emaxpos=0.5 and
ATOMIC_POSITIONS { Angstrom }
O 0.0 0.0 0.0
H 0.77 0.0 0.62
H -0.77 0.0 0.62
the result was: ! total energy = -34.22828551 ryd Total force = 0.063638

Anyway, I'm not very sure if this is the point.

> Maybe the version I used is old.
I think 3.2 (+patches for 3.2.1 and 3.2.2) is the latest.

Giovanni

-- 



Dr. Giovanni Cantele
Coherentia CNR-INFM and Dipartimento di Scienze Fisiche
Universita' di Napoli "Federico II"
Complesso Universitario di Monte S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
Phone: +39 081 676910
Fax:   +39 081 676346
E-mail: Giovanni.Cantele at na.infn.it
Web: http://people.na.infn.it/~cantele