CCL:G: how to choose basis set for optimization of organic molecular

[Herbert Fruchtl <herbert.fruchtl=-=st-andrews.ac.uk>]

 Sent to CCL by: Herbert Fruchtl [herbert.fruchtl ~~ st-andrews.ac.uk]
If you want a series of basis sets with systematic convergence to the
 CBS
 (complete basis set) limit, look at the Dunning basis sets (cc-pvdz,
 cc-pvtz,
 etc.). For a single small molecule you probably don't need extra
 diffuse basis
 sets (aug-cc-pvNz, or "+" with Pople basis sets).
What is probabbly even more important is the method. If you use DFT,
 go for one
 of the more modern meta-hybrid functionals (e.g. M06-2X). The best
 "black box"
 ab initio method for molecules of this size would be CCSD(T), but a
 geometry
 optimization with that can get expensive. Gaussian might not be the
 most
 appropriate program in that case.
 HTH,
   Herbert
 Heng Zhang chemzhh..163.com wrote:
>  Sent to CCL by: "Heng  Zhang" [chemzhh,,163.com]
>  Hi, everyone
>  I am optimizing a molecular C8H8ClNO. There may be two isomers whose energy is
>  very close (less than 0.45 kcal/mol by using 6-31+g*). I want to compute the
>  accurate structure and energy of this molecular by using high precision basis
>  set. Can any one give some suggestions? Thanks.
>  By the way, in that kind of calculation using Gaussian, is there some other
>  things that should be paied attention to.
> Heng Zhang>
>  
>  
 --
 Herbert Fruchtl
 Senior Scientific Computing Officer
 School of Chemistry, School of Mathematics and Statistics
 University of St Andrews
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