[CP2K:6251] QM free energy calculations

[Ari Paavo Seitsonen]

Dear Vladimir,

  Some notes:

 - "PADE" mean LDA which means most likely (strong) over-binding. Why not
using a GGA functional (and/or possibly some explicit treatment of the
London dispersion/van der Waals interactions)?

 - What is the value for CUTOFF that you finally used? 280 Ry is a
historical value plus something used in regtests, not to be recommended in
real calculations. True, LDA is much smoother than the GGA potentials (in
particular BLYP, there one might have to go for 600 Ry or higher, or choose
a smoothing on the XC_GRID).

 - And you use a pseudo potential with only one electron in the valence for
sodium: This is indeed computationally very much nicer than the harrrrrrrd
nine-electron pp (that requires CUTOFF of ~1000 Ry or higher), but I would
not expect it to be accurate, even less because (to my knowledge) there is
no NLCC in this pp.

 - ... and to me it also looks like if the basis set that you are using for
sodium _is_ indeed for the nine-electron pp (very large exponent).

 - In general the DZVP-GTH basis is probably not too accurate, maybe just
at the limit; another option would be the MOLOPT basis sets, there DZVP is
probably sufficient (please refer to the original paper by Joost & Jürg).

  Well, my very quick observations; some one else might extend the list.

    Greetings from Berlin,

       apsi


2015-03-16 20:08 GMT+01:00 Vladimir F <vsfar... at gmail.com>:

> Hello!
> I'm trying to compute hydration free energy of Na+ using AIMD & TI & force
> mixing. The classical computation with SPC water has been successful (400
> kJ/mol against ~414 reference), but the QM one (PADE functional & DZVP
> basis set) has resulted in a as twice as large value 800 kJ/mol. Initial
> cells (water and Na+ in water) were pre-equilibrated (MM and 20 ps QM).
> Productive runs were 1-5 ps before I aborted them. The difference seems to
> be too large to attribute it to short duration only. Please explain, is it
> really so (and I should continue) or is it a mistake in input? Maybe I
>  should apply some corrections to QM results? Also what the "DERIVATIVE
> TOTAL FREE ENERGY" output values mean (I used "DERIVATIVE OF TOTAL ENERGY [
> PARAMETER (k) ]" values)?
> Input files for k=0.5 and dU/dk plots are attached. Gray points are
> extrapolated ones.
>
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-- 
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  Ari Paavo Seitsonen / Ari.P.S... at iki.fi / http://www.iki.fi/~apsi/
  Ecole Normale Supérieure (ENS), Département de Chimie, Paris
  Mobile (F) : +33 789 37 24 25    (CH) : +41 79 71 90 935
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