[ghemical-devel] MM: geometry optimisation gives negative energy???

[Tommi Hassinen]

On Sun, 22 Aug 2004, Parker Jones wrote:

> Dear All,
>
> I am doing geometry optimisation of a small metabolite and end up with a
> minimum of  Energy = -2.19629109 kJ/mol.  All my other calculations have
> given positive energies, so I am a little surprised.
>
> Is this a valid result or could it be a bug?

Hello,

a single MM energy calculated for a single conformer really does not
contain any information ; you only can compare energies calculated for
different conformations for a molecule/compound, and since you only can
compare the energies the zero-level is not important (and usually it is
arbitrary in mol.mech. force fields).

The MM energies calculated for different molecules/compounds are not
comparable.

The QM energies are comparable in some cases ; for example when both
compounds have the same composition (like ethanol CH3CH2OH and
dimethylether CH3OCH3) you should be able to compare their stability from
energy results calculated using identical methods. But I'm not really an
expert of this topic.

Regards,

	Tommi