[Pw_forum] lattice instability and local minimum energy postion

[Axel Kohlmeyer]

On Thu, 31 Jul 2008, Paolo Giannozzi wrote:

PG> 
PG> > [...]  I displaced atoms in the cell according 
PG> > to the eigen-displacements shown above,and got a cubic cell containg 8 
PG> > atoms. But I found the energy rised with the increase of atom 
PG> > displacements, which was totally contrary to what I expected.
PG> 
PG> have you tried the same operation on a simpler system, such as e.g.
PG> GaAs? You should get the phonon frequencies. If you don't, you
PG> may have made a mistake

... not to mention that several of the pseudopotentials from the 
vanderbilt library have problems with getting phonons right. not
sure whether this originates from the definition of the potentials
or the individual parameters chosen. 

i've seen this in several vanderbilt pseudopotential that i created:
they worked fine for all problems that i was using them (MD, geometry
optimization), but when others used them for calculating phonons, they 
turned out to be disastrous.

cheers,
   axel.



PG> 
PG> Paolo
PG> 

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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
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