[Pw_forum] big calculations

[José Alberto Pires Fernandes]

Hello

I'm a newby in calculations with Quantum espresso and I 
think I'm trying to do an impossible task.

I want to do a structural optimization to a cell with 338 
atoms (C, H, N and O) and approximate dimensions of 
15*15*15  angstroms. If I choose Ecut of 18 Ry and a 
kpoint mesh of 1*1*1, with pz-rrkjus pseudopotential the 
optimisation takes 13 hours. With a 2*2*2
kpoint mesh is taking more than 4 days, and hasn't 
finished yet. I believe that for reliable results I need a 
larger Ecut (at least 30 Ry, I suppose) and a number of 
kpoints in the order of thousands, which would imply in a 
calculation of several years.

The cell is a P1 triclinic cell but about 300 atoms are 
fixed, because I only need to do particial optimisation.
So, what can I do to have reliable results in useful time, 
or Quantum Espresso is not suitable for my task?




José Fernandes
Departamento de Quimica
Universidade de Aveiro
3810-193 Aveiro
Portugal
Tel: +351234370720
Fax: +351234370084