[Pw_forum] Display of forces in a pure scf run

[Stefano Baroni]

Cristian,

I may have misunderstood your question, and I confess to know very little of the tstress and tprnfor variables, but forces in pw.x (as well as in most other ab-iition codes) are calculated via the Hellmann-Feynman (HF) theorem. The HF theorem does not require any finite displacements to calculate forces, which are instead used to "move" the ions (either to optimized the geometry or to perform ab-initio molecular dynamics).

If the question was not as simple as the answer actually implies, others may want ti add a less trivial answer ...

Regards,
Stefano

On Dec 6, 2011, at 6:38 PM, Cristian Degli Esposti Boschi wrote:

> Dear developers,
> 
> I have a very basic question, that I had never realized so far and has 
> been asked by a students of mine...
> 
> If I set to .TRUE. the logical fields tstress and tprnfor in the 
> &CONTROL stanza of the pw.x input during a pure scf calculation,
> how are the forces on the ions calculated? What do the displayed
> values represent if only a scf calculation is performed and
> the ionic coordinates are not allowed to change?
> 
> Thanks for your time.
> 
> -- 
> Cristian Degli Esposti Boschi
> CNR-IMM, Sezione di Bologna,
> via Gobetti, 101, 40129, Bologna, Italia
> tel. ++39 051 6399152, fax ++39 051 6399216
> degliesposti -AT- bo.imm.cnr.it
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

La morale est une logique de l'action comme la logique est une morale de la pensée - Jean Piaget

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