[Pw_forum] Display of forces in a pure scf run
On Dec 7, 2011, at 4:23 PM, Cristian Degli Esposti Boschi wrote:
> Dear Stefano and Axel, thanks for your replies. Actually question can be
> rephrased as follows. Thanks to the Hellmann-Feynman theorem
> in order to compute the ionic forces I need only the partial
> derivatives of the total energy with respect to the ionic coordinates
this is not what you need, this is *the* definition of ionic force:
$\mathbf{F}_I=\frac{\partial E}{\partial\mathbf{R}_I}$, with obvious notation
and hopefully no LaTeX errors ...
> (see for instance 7.5 and 7.8 in Payne et al., RMP 64, p. 1045).
> If no real displacements of the atoms are needed to compute
> these derivatives, then I guess that they can be simply calculated
> as expectation values of the Hamiltonian on the current wavefunction
NO: they can be computed as expectation values of the derivative of the
external potential wrt ionic coordinates
> because they can written down once a (parametrized) expression of the
> ionic potential is chosen. Am I right?
I am afraid you aren't, but I am not sure because I could not quite understand
your last statement. There is nothing to choose in the parametric dependence of
the external potential on the ionic coordinates. It depends the way it must
[something like v(r-R), if the notation is clear enough ...], not the way we
choose ...
>
> Thanks for still devoting time to this...
you are most wlcome, as long as we can help ...
S.
---
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
pens?e - Jean Piaget
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[Pw_forum] Display of forces in a pure scf run
Dear Stefano and Axel, thanks for your replies. Actually question can be rephrased as follows. Thanks to the Hellmann-Feynman theorem in order to compute the ionic forces I need only the partial derivatives of the total energy with respect to the ionic coordinates (see for instance 7.5 and 7.8 in Payne et al., RMP 64, p. 1045). If no real displacements of the atoms are needed to compute these derivatives, then I guess that they can be simply calculated as expectation values of the Hamiltonian on the current wavefunction because they can written down once a (parametrized) expression of the ionic potential is chosen. Am I right? Thanks for still devoting time to this... -- 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
[Pw_forum] Display of forces in a pure scf run
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 > ___ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum --- 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 pens?e - Jean Piaget -- next part -- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20111206/7c5ecb28/attachment.htm
[Pw_forum] Display of forces in a pure scf run
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
[Pw_forum] Display of forces in a pure scf run
On Tue, Dec 6, 2011 at 12:38 PM, Cristian Degli Esposti Boschi < degliesposti at bo.imm.cnr.it> 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? > since stefano already answered the first part, here an answer to the second part. the forces and stress reported represent the instantaneous forces and stress of the given conformation. no more, no less. they are uniquely defined by the atom positions and the electronic structure method (and its parameters) and that is it. no motion needs to be considered. ciao, axel. > > 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 > ___ > Pw_forum mailing list > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum > -- Dr. Axel Kohlmeyer akohlmey at gmail.com http://goo.gl/1wk0 College of Science and Technology Temple University, Philadelphia PA, USA. -- next part -- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20111206/0bebe2b6/attachment.htm
