Subject: DFT energies: advice needed

From: Didier MATHIEU <[email-masked]>
Date: Thu, 10 Dec 1998 16:23:56 +0000

Hello,

A student of mine developped a set of correction terms to convert DFT
energies (BP/DN**//MMFF) into heats of formation at 298K. Different
correction terms have been determined for different atoms with different
coordination numbers, as done in earlier work:

  Habibollahzadeh et al. J. Comp. Chem. 16 (5) 654-658 (1995)

We attempted to select carefully the compounds in the calibration set.
As a result, a systematic trend was observed: for a given element with
atomic number Z, the associated correction term gets higher as the
coordination number gets lower. 
!! (!def.: these correction terms are ADDED, not substracted, to total
energies including DFT + thermal + PV).

In other words, larger correction terms are required on going from
saturated to unsaturated system.

Of course it might be a matter of chance, buit I wonder whether it could
be explained in light of the known systematic errors on DFT
(Becke-Perdew86) energies, despite the use of MMFF geometries which can
hamper the interpretation of such results.

According to such results it would seem that DFT energies are more
overestimated - or less underestimated - for saturated compounds.

Assuming larger correlation energies when more multiple bonds are
present, this would amount to saying that BP86 tends to overestimate
correlation, but I am not aware of such a trend... 

I find DFT energies more difficult to interpret that ab initio values,
and any help would be greatly appreciated.

Thanks & regards
-- 
Didier MATHIEU
CEA - Le Ripault, BP 16
37260 Monts (France)
Tel. 33(0)2.47.34.41.85