[Pw_forum] constraints in the ionic dynamics

[Stefano Baroni]

Dear Hanghui: I think this is just because of periodic boundary  
conditions. Thinking in 1D, as always, helps. Suppose you have a cell  
of size L. You place one atom, say A, at the origin (x=0), and  
another one, say B, at x=2/3 L. What will be the distance between A  
and B? Not 2/3 L, but L/3!

Hope this helps
Stefano Baroni

On Nov 9, 2007, at 4:02 AM, alan chen wrote:

> To whom it may concern,
>       I am trying the CONSTRAINTS card for ionic dynamics. One of  
> the 'constr_type' is 'distance'. However, if the distance of the  
> two atoms I want to fix is larger than 'dmax' (dmax is half the  
> shortest edge of the cell), the code crashes.
>       I would like to ask why we can not fix two atoms whose  
> distance is NOT larger than 'dmax'? Does the algorithm not work  
> well if the distance of two atoms to be fixed is too large?
>       Thank you very much.
>                                                                        
>                                                       Hanghui
>                                                                        
>                                               Department of Physics,
>                                                                        
>                                                        Yale University
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center -  
Trieste
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