[Pw_forum] van der waals force

[Lorenzo Paulatto]

In data 13 maggio 2009 alle ore 15:59:05, Mehrnaz Anvari  
<anvari_meh at physics.iust.ac.ir> ha scritto:
> Is it correct or some correction is implemented in Q-E.

Local density aproximation does not reproduce VdW bonds correctly, it can  
either overbound (like plain LDA usually does) or underbound (more common  
for gradien-corrected functionals). Yet, if you know the limits you can  
take your data with a grain of salt. At the moment QE does not implement  
any VdW correction by default, exact-exchange corrections are implemented  
but not compiled by default.

> Also during my calculation I realized that when I increased the value of
> (celldm(3)=3-10) to attached isolated plane for graphene, there isn't any
> noticeable flactuations in total energy, but the values of subenergies  
> such as hartree, ewald energy,...change extremely,I mean these terms  
> become large and  I think it is not logical.

I don't see the problem, if you have a look a the definitions of the Ewald  
summation for the energy <http://en.wikipedia.org/wiki/Ewald_summation>  
you can see it is perfectly normal. Of course the total energy has to  
converge for increasing size of the cell.

regards

-- 
Lorenzo Paulatto
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