Re: CCL:Crystal structure vs geometry optimised structure

2003 July 23
 Hello,
 It is probably not a good idea to use experimental geometries for freq calc's,
 because
 freq calc's are strictly valid only at the geometry obtained by the same method
 as the
 freq calc. Thus a B3LYP/6-31G* freq calc should be done on a geometry obtained
 by a
 B3LYP/6-31G* optimization. Since your freq jobs are likely to be about as long
 or longer
 than any geom optimizations, you probably won't lose very much time by doing the
 optimizations first.
 Another problem with crystal structures is that they may differ significantly
 from the
 isolated molecule structures that I presume you will use for your freq jobs (you
 won't
 use a crystal lattice as input, I guess); but you took account of this by saying
 "where
 the DFT...is in excellent agreement..."
 It would be interesting to do some freq calc's on both DFT-opt and experimental
 structures and see how much they differ. But conventional wisdom is that the
 DFT-opt
 structures are the ones to use.
 If you didn't want freqs, just relative energies of isomers, you could do
 single-point
 calculations (much faster than optimizations) on the experimental structures.
 Again, it
 might be interesting to compare relative energies based on the experimental and
 the
 DFT-opt structures.
 E. Lewars
 ==========
 noel.oboyle2/at/mail.dcu.ie wrote:
 > Hello CCLers,
 > I intend to carry out DFT calculations on inorganic complexes. In some
 cases
 > I have crystal structures available.
 >
 > Is it better to use crystal structures as a starting point for energy and
 > frequency calculations, or to use DFT-optimised structures? (where the
 DFT-optimised
 > structure is in excellent agreement with the crystal structure)
 >
 > Thanks,
 > Noel O'Boyle
 > Han Vos Research Group,
 > Dublin City University.
 >
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