From mutz@ifp.mat.ethz.ch Mon Apr 24 13:33:14 1995 Received: from bernina.ethz.ch for mutz@ifp.mat.ethz.ch by www.ccl.net (8.6.10/930601.1506) id NAA13089; Mon, 24 Apr 1995 13:19:29 -0400 From: Received: from dove (actually dove.ethz.ch) by bernina.ethz.ch with SMTP inbound; Mon, 24 Apr 1995 19:19:12 +0200 Message-Id: <9504241719.AA14505@dove> Received: from ifp.mat.ethz.ch (erne) by dove.ethz.ch id AA14505; Mon, 24 Apr 95 19:19:27 +0200 Received: by erne.ethz.ch; Mon, 24 Apr 1995 19:20:51 +0200 To: Simon Collins Cc: chemistry@ccl.net, mutz@ifp.mat.ethz.ch Subject: Re: CCL:Charges/bond-breaking In-Reply-To: (Your message of Mon, 24 Apr 95 16:17:13 N.) Date: Mon, 24 Apr 95 19:20:51 +0100 Simon Collins wrote: ------------ Dear All, I am presently interseted in the varience of charge with energy for the heterolytic dissociation of various simple acids, such as HF. I am trying to do this by simply doing single point energies at varying increasing bond lengths, but the molecule splits up homolytically into the radicals. Is there any way round this problem? ------------- Dear Simon, you might do just the opposite of what you did: Start with the separated ions and approach them step by step, instead of breaking up the molecule. Of course, as you approach them, the charge will be redistributed at some point to give more or less the charge distribution of the molecule, but this is what you wanted to model, after all. BTW, there are two different ways to split up a molecule like HF homolytically. The molecule HF in its ground state has all its electrons paired, so it is a singlett. If you separate the two radicals by some distance (e.g. you break the bond), each of the two atoms gets one unpaired electron, and it has to be expected from Hund's rule that the triplett configuration has lower energy. So somewhere midway between the intact molecule and the separated radicals there must be point where the energies of the singlett and triplett state are degenerate in the approximation of Hartree-Fock theory. I don't know about HF, but it might well be that the system undergoes *two* rather than one transition if you move the ions towards each other: Ionic --> Triplett --> Singlett . One last word of caution: The transition point from Triplett to Singlett state is a configuration where there is a large coupling between the movements of the electrons and that of the nuclei (a small movement of the nuclei makes the electron configuration change *qualitatively* !). This means that at this point, one of the most fundamental approximations in electronic structure theory, the Born-Oppenheimer Separation obviously breaks down. It is anything but clear how such systems should be treated (Anybody out there on CCL has any experience in the field, perhaps ? ) Greetings from Zurich -Marcel Utz. ------------------------------------------------------------------------------ Marcel Utz phone: +41 1 632 5672 Institute of Polymers fax: +41 1 632 1096 ETH-Zurich CNB E 98.2 CH-8092 Zurich, Switzerland email: Marcel.Utz@ifp.mat.ethz.ch ------------------------------------------------------------------------------