Re: CCL:colour of molecules in the solid state

 Anik Peeters wrote:
 > Dear CCLers,
 > I am studying molecular crystals using the supermolecule approach:
 > a cluster of 10-15 molecules is described by the wave function
 > and this cluster is surrounded by point charges simulating the other
 > neighbours. The level of theory we are using is Hartree-Fock.
 > The molecule I am studying right now has been observed in three different
 > polymorphic forms. One has a white colour, another one is yellow and a
 > third one is light yellow. I have been trying to relate the yellow and
 > light yellow colour to the band gap between the HOMO and LUMO.
 Anik- I doubt whether the "band gap" has anything to do with the
 you observe. More likely they are either charge transfer transitions or
 possibly Davydov splittings of the exciton states.
 Before you do anything fancy why don't you visually looked at the
 structures to see if there are any molecular interactions which would
 suggest charge transfer transitions?
 Jerry Perlstein
 Center for Photoinduced Charge Transfer
 Department of Chemistry
 University of Rochester
 Rochester,NY 14627-0216
 Tel: (716)275-2511
 FAX: (716)242-9485
 perlstein # - at - #