Re: CCL:Charges/bond-breaking



 Simon Collins wrote:
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 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?
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 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.
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 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 # - at - #
 ifp.mat.ethz.ch
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