Subject: CCL: QM/MM cutoffs

From: "Nicolas Ferre'" <ferre^at^unisi.it>
Date: Fri, 30 May 2003 16:41:48 +0200

Hi folks,

I'm currently implementing a simple QM/MM molecular dynamics approach,
but I have some doubts about the right way of dealing with the
electrostatic interactions between the quantum distribution and the
classical point charges. Using periodic boundary conditions and the
minimum image convention, I have to define a cutoff distance : a
site-site interaction is discarded if the site-site distance is greater
than this value. OK for a classical simulation.
Now if part of the system is treated quantum mechanically, the
definition of some QM sites is non-obvious : the QM wavefunction is
delocalized on the whole QM subsystem.
The simplest idea would be to select the center of mass of the QM
subsystem as a unique site only to choose MM point charges interacting
with the QM subsystem, but if the QM molecular shape is not spherical,
this solution is not convenient (a QM atom located at one end of the QM
subsystem could not interact with the closest images of some MM point
charges).
Thus I'd like to hear about your solutions/experiences/references to
solve this problem, taking in mind that I don't want to approximate (for
the moment) the QM electronic distribution (no fitted multipoles ...) I
know about Ewald sums for QM/MM interactions but I'd prefer to start
with a rather simplest approach.

Best regards,

Nicolas

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Nicolas FERRE' (PhD)
				 phone/fax : +39-0577-234278
Dipartimento di Chimica
Universita` di Siena             mailto:ferre^at^unisi.it
via Aldo Moro
53100 SIENA (Italia)             http://ccmaol1.chim.unisi.it/
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~