[CPMD-list] Non-physical aqueous Na+ behavior

[Dave Sherman]

Dear CPMD list,

I'm doing CP MD simulations on aqueous solutions (25 H2O, 1 Sn,  4 Na, 
5 Cl) using ultrasoft pseudopotentials and a cutoff of 25 Ry.  The 
simulations are run using a Nose thermostat set to 600 K a and the 
volume corresponds to about 10 kbar pressure.  Everything is great; all 
of the bond lengths and transient complexes are reasonable (good 
agreement with my static all electron calculations using ADF and 
experimental results) and total energy is conserved.  After 6 ps (50000 
steps), however,  I start to see a sodium-sodium contact ion pair 
forming in a bound state ( you can see the little oscillations..) with 
a Na-Na separation of < 2.2 A.  I'm assuming this is  non-physical, of 
course (or it would make the cover of Science..).  What could possibly 
cause this?  Is there some kind of "wavefunction collapse"  or 
pathology with the Na+ pseudopotential going on? (I'm using the 
Vanderbilt group Na potential with Ceperly-Alder exchange and semicore 
sp states..).  Is it possible that, when the Na+ cations get close 
enough, each Na+ sees the psuedopotential of the other and gets bound 
to it?   I believe I'm still on the Born-Oppenheimer surface;  Ekinc is 
pretty stable at 0.15 AU and EHAM is very stable  (to within 0.001 AU). 
  I'm using CPMD 3.7.

Thanks for your help!

Dave Sherman

David M. Sherman
Professor of Geochemistry
Department of Earth Sciences
University of Bristol
Bristol BS8 1RJ UNITED KINGDOM
Phone: 44-(0)117-954-5446
http://mineral.gly.bris.ac.uk