[ghemical-devel] MM/QM: assumptions

[Tommi Hassinen]

On Tue, 24 Aug 2004, Parker Jones wrote:

> Thanks for the helpful response, Tommi.  It appears I have been violating
> the assumptions for MM calculations.  I'd like to calculate the energy of
> the LHS and RHS of a reaction where substrates and products are small
> metabolites.  You mention that QM energies are comparable in some cases.

LHS = left hand side, and RHS = right hand side of an equation, right?

> In that case, can one simply sum the QM energies of the molecules of each
> side?  Any pointers to the assumptions of QM calculations would be much
> appreciated.

Yes, I think one can sum the QM energies of each side, and subtract them
to get the (approximate?) delta-E for the reaction. Please note that

1) make sure the QM method used is "size-consistent" ; the energy for 2
separate non-interacting molecules should be 2x energy of a single
molecule. Most methods are size-consistent, but it's good to check this.

2) the results describe the reaction in gas-phase conditions. In the real
reaction some/all of the reactans/products can be liquids/solids but this
is not taken in account, but the result is a gas-phase result. Solvation
can favour reactans/products or both. Perhaps some programs/methods can
approximate this as well.

I can't give any references into this, but try looking for thermodynamic
applications of QM methods.

Regards,

	Tommi