[Pw_forum] Suitable pseudopotentials for LiH..H2O

[Stefano Baroni]

By comparing MP2 all-electron calculations performed using localized basis sets and free boundary conditions, with DFT pseudopotential calcs, done with plane waves and periodic boundary conditions you have succeeded in making the two calculations as heterogeneous as possible. Well done (converged) calculations should not depend on the basis set, pseudopotential, or boundary conditions, but they certainly will on the level of theory you use. Getting very different results with MP2 and DFT should make you suspect that some kind of inter-molecular correlation effects may play an important role. The first thing I would do is to repeat the calculation with localized basis sets at the DFT level, and see if the structure you obtain matches the one you get with QE. SB
 
On Jun 20, 2011, at 6:54 AM, psavita at crlindia.com wrote:

> Dear Prof. Ghosh,
> 
> As per your suggestion, I performed the structure relaxation of LiH--H2O system using a bigger cubic box of dimension 50 a.u.
> and I still find H2 abstraction instead of an intramolecular dihydrogen bond.
> Do you think the box is still inadequate?
> For such systems, would it be better to put some constraints initially?
> How do we do this in QE?
> 
> Thanks for your suggestion and best regards,
> 
> Savita Pundlik
> Computational Materials Applied Research Group
> Computational Research Laboratories Ltd.,
> Taco House, Damle Path, Off Law College Road
> Pune - 411004, India.
> 
> 
> 
> -----pw_forum-bounces at pwscf.org wrote: -----
> 
> 
> 
> 
> 
> 
> Dear Savita,
> 
> I am not an expert on these type of systems.......however just a small piece
> of advice.....a single molecule of water has a huge dipole moment.....so a
> box size of 25 bohr may not be sufficient, until and unless binding to LiH
> quenches the dipole moment......so you may like to check whether your box is
> sufficiently big.....usually for a single molecule of water one uses about
> 35-40 bohr box or can use dipole correction.
> 
> Prasenjit
> 
> On 17 June 2011 11:42, <psavita at crlindia.com> wrote:
> 
> > Hello,
> >
> > I have been trying to get the optimum structure of LiH..H2O that has an
> > intramolecular dihydrogen bond, using pwscf.
> > I could get the proper structure earlier using mp2 theory and 6-31++g
> > basis.
> >
> > For pwscf, the species is enclosed in a cubic box of 25 bohr for gamma
> > point calculation.
> > I tried the pz lda, pbe gga as well we blyp PS for the atoms in this
> > system, however, the first and the last
> > lead to H2O..LiH, the latter LiH being linear while pbe gives LiH + H2, but
> > no dihydrogen bond. Is there any other PS
> > that may lead to proper structure? Or is it that plane wave approach does
> > not suit systems of this kind?
> >
> > Thanks in advance and best regards,
> >
> > Savita Pundlik
> > Computational Materials Applied Research Group
> > Computational Research Laboratories Ltd.,
> > Taco House, Damle Path, Off Law College Road
> > Pune - 411004, India.
> >
> >
> >
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> >
> >
> 
> 
> -- 
> PRASENJIT GHOSH,
> Assistant Professor,
> IISER Pune,
> First floor, Central Tower, Sai Trinity Building
> Garware Circle, Sutarwadi, Pashan
> Pune, Maharashtra 411021, India
> 
> Phone: +91 (20) 2590 8203
> Fax: +91 (20) 2589 9790
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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)

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