[CPMD-list] Liquid simulation using CPMD code

[Juerg Hutter]

Hi

If you are not using the

SYMMETRY
  0

options your calculations are using periodic boundary
conditions (PBC). PBC are imposed within CPMD for
all calculations. However, the particle positions
are not folded back to the original computational
box. The reason for this is that most people prefer
to have "smooth" trajectories without jumps of
particles. This allows for easier tracking of
special particles and nicer graphics. In addition
it is easy (with a little script) to apply PBC
yorself if needed.

Sodium is a metal, and therefore missing an important
feature that allows for stable CP dynamics: the band gap.
Using Nose thermostats (on electrons and ions) it
might still be possible to perform meaningfull CP
simulations (see P. Blochl, M. Parrinello, PRB (45) 9413 (1992)).
The choice of parameters for the thermostat will be
nontrivial, highly system dependent and require extensive
testing.
Without thermostats you will have strong coupling between
electronic degrees of freedom and ionic dof. Adiabaticity
is not maintained and a steady increase of the ficticious
kinetic energy will occur.

regards

Juerg


----------------------------------------------------------
Juerg Hutter                   Phone : ++41 1 635 4491
Physical Chemistry Institute   FAX   : ++41 1 635 6838
University of Zurich           E-mail: hutter at pci.unizh.ch
Winterthurerstrasse 190
CH-8057 Zurich, Switzerland
----------------------------------------------------------


On Wed, 4 Dec 2002, niharendu choudhury wrote:

>
> Dear Prof./friends,
> I am trying to simulate bulk liquid sodium (Na) using
> CPMD with 108 particles and supposing that periodic
> boundary condition is in-built in the program. But
> after 20000 MD steps of 10 a.u. each, I found some
> particles are away from the central simulation box,
> which is originally ~16 Angstorm long.
>
> Why it is so if periodic boundary conditions (PBC) on
> particle coordinates are imposed in all the three
> directions?
>
> Also I found electron energy is increasing
> continuously and it is in the range of 0.07 a.u. at
> the end of 20000 steps.
>
> Is the dynamics running properly?
> Is the PBC autometically imposed by the program or I
> have to give some command in the input file to impose
> it?
> Your reply will be highly appreciated.
> Thank you.
> Dr. Niharendu Choudhury
>
>
>
>
>
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