[Pw_forum] The cell blowing up in MD simulation

Tian Lan tianlan at caltech.edu
Wed Sep 26 19:19:14 CEST 2012

Hi sir,

Thank you. I am trying to keep every setup the same in CP.x and Pw.x as you
suggested. You mentioned that the k-points should be the same. But I cannot
find the input card like &K_Points in cp.x, how could I control the k
sampling? I only find the charge meshing commands like nr1...in &System. So
I guess cp.x used some default setup or some different algorithm in MD


On Fri, Sep 14, 2012 at 7:30 AM, Paolo Giannozzi <giannozz at democritos.it>wrote:

> On Thu, 2012-09-13 at 15:36 -0700, Tian Lan wrote:
> > But it seems that the energies that both codes obtained agree well. I
> > pasted the results in the following. The first one is the result from
> > vc-relax operated on one unit cell, the total energy is -358.1 Ry,
> > this is very close to my original setup with scf calculation. The
> > second one is from cp.x operated on 2 by 2 by 2 supercell, with fixed
> > atom positions(ion and cell_dynamics are set to 'none'), the energy is
> > -1410 au==-2820 Ry. I don't know whether this is a big difference
> the energy is -1431.80272 Ha or so, i.e. -1431.80272/8 Ha per cell,
> i.e. 357.95Ry. Close (358.097) but not very close. Note however that
> you need to compare equivalent results, i.e. same cell, same k-points,
> or a cell and a supercell but with equivalent k-points. Also note
> that the treatment of the augmentation charge via the "small boxes"
> requires some testing in order to find a suitable value for input
> variables nr1b, nr2b, nr3b. The resulting pressure may also change
> quite a bit.
> > I don't quite get your point about the default positions of atoms in
> > pw and cp. In both codes, I used 'crystal'
> perfect: it is of no concern for you. If you do not specify anything,
> cp will assume borh radii and pw lattice parameters, or something
> like this, as explained in the documentation
> P.
> --
> Paolo Giannozzi, IOM-Democritos and University of Udine, Italy
> _______________________________________________
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> Pw_forum at pwscf.org
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Lan, Tian
Ph.D. Candidate, Department of Applied Physics and Materials Science
California Institute of Technology,
Caltech M/C 138-78, Pasadena, CA, 91125
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