[Pw_forum] vc-relax of graphite

[Paolo Giannozzi]

On Mar 12, 2007, at 13:18 , li niu wrote:

> When I used a symmetry-conserving algorithm,  cell_dynamics = 'damp- 
> w' ,
> I still met the same error :
> ---------------------------------------------------------------------- 
> ---
> from checkallsym : error #         3
>      not orthogonal operation
> ---------------------------------------------------------------------- 
> --

then break the initial symmetry by displacing an atom a little bit

> And when using  "nosym = .true.", I met a warning: [...]
>      WARNING: integrated charge=    10.59801382, expected=     
> 16.00000000

you did something wrong

> Now I don't know how to  optimize lattice constant of the bulk  
> graphite.

do it the good old way: fix c to the experimental value (with DFT you
get bogus values in graphite anyway) and optimize only a. Or, if you
are really curious to see what the theoretical value of graphite look
like, choose a few volumes, optimize c/a for each of them, then do
a E(V) fit. Using variable-cell MD to find optimized lattice constants
for graphite is like using a gun to kill a mosquito

P.
---
Paolo Giannozzi, Democritos and University of Udine, Italy