[Pw_forum] The scf can't converge

[Axel Kohlmeyer]

On Sun, 2009-05-17 at 23:18 +0800, wangweiphysics at yahoo.com.cn wrote:

[...]

dear wang wei,

> I have another queation:
> I don't know the difference between atomic position in units of alat 
> and in crystal coordinates. When I use the crystal coordinates, scf 
> can converge easily, but the atom positions in the output file are
> different from those in the input file. The same atom positins obtain

i doubt that. please provide an example. have you compared input and 
output coordinates in xcrysden?

> using atomic position in units of alat but can't converge.I am very 
> puzzled!

you see, this is _exactly_ the reason, why you should post your
complete input in the first place. bad initial atom positions is
one of the possible reasons for problems with SCF convergence, as
not physically meaningful atom positions will make a convergence
very difficult. 

the difference of alat coordinates and crystal is very simple:
alat are cartesian coordinates in units of the lattice constant,
whereas crystal are fractional positions within the unitcell.
for a primitive cubic lattice, they are identical, but i guess
this is not what you are using.

cheers,
  axel.

> Wang wei,
> Southeast University, Nanjing, China
> 2009.5.17




         
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-- 
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Axel Kohlmeyer   akohlmey at cmm.chem.upenn.edu   http://www.cmm.upenn.edu
   Center for Molecular Modeling   --   University of Pennsylvania
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