[molpro-user] dispersion question

[Glen Jenness]

Dear Molpro users,
I've been doing DF-DFT-SAPT calculations on coronene-water and I noticed
something strange when I go from augmented double zeta to augmented triple
zeta.
For double zeta, I get dispersion (and exchange) of ~-4.7 and 0.6 kcal/mol
(supermolecular calculations that our group did a couple years ago suggest
this value should be between -4 and -6 kcal/mol).  However when I move to
triple zeta, both exchange-dispersion and dispersion drop go to something
like O(10^-3), which is way low.
I've experimented with averaging teh coupled dispersion with the uncoupled
dispersion, and that yields a result of ~-4.8 kcal/mol (for exchange it
yields ~.5 kcal/mol).
For a table of the results, please see:
http://www.cmms.pitt.edu/~glen/index.php/Main_page/cw/df-dft-sapt_cw
I was wondering if anyone had any thoughts on this?
Thanks in advance,
Glen Jenness
Jordan Group
Department of Chemistry
University of Pittsburgh