Subject: Re: CCL:MM vs MD

From: Anton Feenstra <feenstra.-at-.chem.vu.nl>
Date: Mon, 01 Dec 2003 07:51:45 +0100

Arvydas Tamulis wrote:

> Dear Prof. Telkuni Tsuru and other Netters,
> 
> I am working with quantum mechanical Hartree-Fock and Density Functional
> Methods. Would you be so kind and explane me more detail what it is the
> difference between methods:
> 1) MM, describing with force fields and bonding parameters and
> 2) MD, describing with potential functions ?

In the terms you use, there is no difference. Both use *potential functions*
to describe *bonding paramaters*, together these are called a *force field*.

One can view Molecular *Dynamics* as a special case of Molecular *Mechanics*
where dynamics comes in through assigning velocities to all atoms (and hence
a system *temperature*), and integrating the (Newtons) equations of *motion*.

> Which is more exact in comparison with quantum mechanical methods?

IMHO, that is not a sensible question. The realm, if you will, of both methods
(i.e. 'QM' vs 'MM/MD') and possible applications, is so different that there
is no sensible way in which to compare them. Furthermore, many parameters in
the forcefields have their basis in QM calculations, so that MM/MD can be said
to be derived from QM, which implies it is less accurate. But on the other
hand, the MM/MD parameters are also based on (other) experimental knowledge
(often thermodynamical parameters like heat of vaporization) which make them
on a certain level of application at least in principle more correct (and
at the same time they are, obviously, empyrical parameters with limited
applicability).

-- 
Groetjes,

Anton
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|  _   _  ___,| K. Anton Feenstra                                     |
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