Subject: Re: CCL:G:Calculate Heat of formation ?

From: Didier MATHIEU <[email-masked]>
Date: Tue, 01 Dec 1998 17:33:06 +0000

Elmar Gerwalin wrote:
> 
> Hi
> I want to use Gaussian94 to calculate the heat of
> formation for organic compounds.
> 
> Do I have to perform a frequency calculation to get the
> Zero point vinbration energy to get correct results ?

Concerning calculation of Heats of formation and other thermochemical
quantities, there is a very useful (to me) recent book, in which a wide
variety of methods, from purely empirical to very high level ab initio,
are discussed:

   Computational Thermochemistry
   ACS Symposium Series No 677
   K. Irikura and D.J. Frurip (eds).

This book is available from the Amazon bookshop:

   http://www.amazon.com

> Or is there a better way to get the values ?

It depends on a number of factors such as the target accuracy, the size
of the molecules considered... If you can afford high-level ab initio
techniques of course calculation of zero-point energy is recommended,
and also others less significant contributions (thermal vib+rot+trans
and delta(PV)...) easy to obtain. My version of Gaussian is G92 but I
think in Gaussian94 calculation of formation enthalpies is implemented
in an automated way (G2 or CBS models).

If more efficient techniques are required, you need "atom equivalents":
see S.J. Mole et al. J. Phys. Chem. 100, 14665 (1996) and ref. therein.

A set of atom equivalents for C,H,N,O,F,Cl compounds has been derived by
Habibollahzadeh et al. J. Comp. Chem. 16, 654 (1995).

To come back to the original question, I believe than explicit
calculation of frequencies is useful only at the highest levels, because
using HF or popular DFT functional the errors stem mostly from the
equilibrium energies (even after correction through atom equivalents)
rather than from the neglect of ZPE and thermal contributions.  Maybe
with B3LYP and large basis set (Mole et al.) introducing explicit
calculation of the frequencies might be useful, but it is doubtful to
me. 

For instance, in JCC 16, 654 (1995) all contributions are calculated
explicitly. However, it yields no improvement over the more usual method
were everything is dumped into atom equivalents exepted the DFT energy.
We tried both schemes and obtained very similar results.

To save CPU time, you can simply perform single point energy
calculations on geometries optimized with accurate molecular mechanics
force field such as MMFF. We developed a set of atom equivalents
suitable for BP/DN**//MMFF calculations. There were obtained from a
calibration set of 84 molecules and tested on the panel of JCC 16, 654
(1995): we obtained the same accuracy of the original method at a much
reduced cost. I can send the group equivalents.

However, one must be careful with the environments of the atoms. For
instance we found out that the atom equivalents for F bonded to C CANNOT
be used for F bonded to N, at least at the BP/DN**//MMFF level.
Moreover, MMFF wrongly predicts nitro groups to be perpendicular to the
ring plane for nitroaromatics!

In JCC 16, 654 (1996) it is obvious that uncorrected theoretical
enthalpies are very poor. For fast calculations we use the PIMM model
available from the university of Darmstadt:

   http://tutor.oc.chemie.th-darmstadt.de/TZ/AKLindner/pimm_e.html

To my experience, the PIMM method is faster and more reliable than both
AM1 (significantly) and PM3 for energetic  materials, although it was
not designed for this specific application.

Here is a comparaison of performance for ~100 molecules, most of them
staken from JCC 16, 654 (1995):
values reported = average absolute deviation and max. abs. dev.
(kJ/mol):

   PIMM method           19.5    13.3
   BP/DN**//MMFF (*)     12.5    59.6
   BP/DN**//MMFF (**)    12.3    64.8
   BP/6-31G(d,p)         13.3    43.8

(*) freq. calculated explicitly, hence VIB+ROT+TRANS+D(PV)
(**) no explicit calculation of the above contributions

Regards

Didier

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