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

[Ulf Ekström <ulfek-x-few.vu.nl>]

 Sent to CCL by: =?ISO-8859-1?Q?Ulf_Ekstr=F6m?= [ulfek-,-few.vu.nl]
 > 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.
 There is no dft method that I am aware of that is accurate to 0.45 kcal/mol,
 and at this level you should also include dispersion and vibrational
 effects (and thermodynamics!). This talk by T. Helgaker gives a good
 overview of the
 basis set and correlation effects: http://folk.uio.no/helgaker/talks/esor.pdf
 Of course just because you might not be able to say which of the two isomers
 have the lowest energy doesn't mean that your structures will be inaccurate,
 they may well be ok with standard dft methods, specially if your molecule is
 not too floppy. Use a triple-zeta basis set in the final stages of your geometry
 optimization.
 Sincerely,
 Ulf Ekström