From chemistry-request #at# ccl.net Fri May 30 10:50:37 2003 Received: from cuces.unisi.it (cuces.unisi.it [193.205.4.2]) by server.ccl.net (8.12.8/8.12.8) with ESMTP id h4UEoagC015947 for ; Fri, 30 May 2003 10:50:37 -0400 Received: from CONVERSION-DAEMON.cuces.unisi.it by cuces.unisi.it (PMDF V6.1-1 #30546) id <01KWICZWFMOG000S76^at^cuces.unisi.it> for chemistry^at^ccl.net; Fri, 30 May 2003 16:50:30 +0100 (MET) Received: from unisi.it (ccmaol10.chim.unisi.it [172.16.29.96]) by cuces.unisi.it (PMDF V6.1-1 #30546) with ESMTPA id <01KWICZVP9QS000WYZ^at^cuces.unisi.it> for chemistry^at^ccl.net; Fri, 30 May 2003 16:50:30 +0100 (MET) Date: Fri, 30 May 2003 16:41:48 +0200 From: "Nicolas Ferre'" Subject: CCL: QM/MM cutoffs Sender: ferre^at^unisi.it To: chemistry^at^ccl.net Message-id: <3ED76DAC.EB291CEB^at^unisi.it> Organization: Universita` di Siena MIME-version: 1.0 X-Mailer: Mozilla 4.78iC-CCK-MCD [en_US] (X11; U; AIX 5.1) Content-type: text/plain; charset=us-ascii Content-transfer-encoding: 7bit X-Accept-Language: en 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/ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~