CCL:G: Effect of basis set on frequency calculations with g03

From: "Rajendran Arivazhagan" <raji(-)anal.chem.tohoku.ac.jp>
Subject: CCL:G: Effect of basis set on frequency calculations with g03
Date: Tue, 17 Oct 2006 23:41:46 -0400

 Sent to CCL by: "Rajendran  Arivazhagan"
 [raji[a]anal.chem.tohoku.ac.jp]
 Dear CCL Members,
      I am doing frequency calculation for protonated organic molecule using g03.
 My molecule have two protonation sites, say for example site1 and site2. Freq
 calculation with B3LYP/6-311+G(d) gave an imaginary/negative frequency for site1
 and all real frequencies were observed for site2. My input file looks like:
 B3LYP/6-311+G(d) SCF=Tight test Opt Freq Pop=full gfprint SCRF=(CPCM,
 Solvent=Water)
 But the results are completely opposite with B3LYP/3-21G and here negative
 frequency was observed for site2 and all real frequencies were observed for
 site1. Input file looks like:
 B3LYP/3-21G SCF=Tight test Opt Freq Pop=full gfprint SCRF=(CPCM, Solvent=Water)
 So which result I can believe and where is the problem? Basis set with
 polarization and diffuse functions are not suitable for frequency calculations?
 Any suggestion is most welcome. Thank you very much.
 A.Rajendran.