[Pw_forum] vc-relax versus scf calculations on various cells

[vtmtrinh at caltech.edu]

Dear All,

I am running a test calculation on a cell with 64 Si atoms.  I first run a
series of scf calculations of different cell sizes (the atoms position
coordinates are also generated accordingly with cell dimensions by my
code).  I got the following results:

a (A)             Total E(Ry)
10.775	        -508.1653034
10.770	        -508.1653391
10.76932	-508.1653368
10.765	        -508.1653067
10.76	        -508.1652012

>From the above results, it looks like to me I would have a min E close
to/at a=10.77A (plotting E vs a give me a nice parabola curve).  I then
rerun the structure at min E (i.e. a=10.77) using vc-relax.  I obtained
the following:

CELL_PARAMETERS (alat)
   1.000000000   0.000000000   0.000000000
   0.000000000   1.000000000   0.000000000
   0.000000000   0.000000000   1.000000000


alat = 20.3289 au (or 10.75760138 A),  E=-508.1650690 Ry

Comparing to the results by scf calculations, I see the result from
vc-relax and scf calculations are not consistent at all.  I thought they
should. However, the total stress computed for the scf calculation at min
energy is still large (P = -3.36 kbar), while the stress calculated by the
vc relax is small  P = -0.04 kbar.  I also noted that the cell size at
which the system has a min stress (i.e. almost zero stress) given by scf
calculations does not coincide with the cell size at which the the system
has a minimum energy (min of the parabola).  I thought that at the
optimized cell size, both stress and energy have to be minimum. I don't
understand why there is such a difference.  Am I wrong in the way I was
doing?

Thank you very much,

Trinh Vo
JPL/Caltech