CCL:G: Method for Frequency Calculations

 Sent to CCL by: "Abrash, Samuel" [sabrash(-)richmond.edu]
 If you choose a DFT Method with a biggish basis set, you can get within
 a few % of literature values.  People who do matrix isolation
 spectroscopy and commonly use Gaussian for vibrational spectrum
 calculations usually use B3LYP/6-311++G**.
 You can also get a sense of the accuracy by going to CCCBDB and going to
 scaling factors.  The closer the scaling factor is to one for a
 combination of theory and basis set, the more accurate it is.
 HF performs worse than MP2 as far as absolute frequencies go, but tends
 to have frequency patterns that match experiment better.  MP2 gives
 smaller scaling factors but can be unpredictable regarding whether
 frequencies are over or underestimated.
 Best regards,
 Sam Abrash
 Samuel A. Abrash
 Department of Chemistry
 University of Richmond
 Richmond, VA 23173
 Phone:  804-289-8248
 Fax:  804-287-1897
 E-mail:  sabrash#%#richmond.edu
 Web-page:  http://oncampus.richmond.edu/~sabrash
 "Rabbi Yitzhak said:  At the time God created the world and desired to
 reveal the depth of His being from out of the hidden, the light came
 > from the darkness and they were joined together.  Because of this, out
 of darkness came the light and out of the hidden came the revealed and
 out of the good came evil and out of mercy came severe judgement, and
 everything is intertwined  with everything else...the good inclination
 and the evil inclination, the right and the left." - The Zohar
 -----Original Message-----
 > From: owner-chemistry#%#ccl.net [mailto:owner-chemistry#%#ccl.net]
 Sent: Friday, April 06, 2007 11:15 AM
 To: Abrash, Samuel
 Subject: CCL: Method for Frequency Calculations
 Sent to CCL by: "Soren Eustis" [soren__jhu.edu]
 Disregarding any concerns with processor time and assuming extensive
 basis
 sets, what is generally accepted to be the most accurate method for
 frequency calculations of small (non-metallic) molecules?  I have heard
 MP2
 is not that great, but HF and DFT perform better?  Am I on the right
 path
 here?http://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp-:-//www.ccl.net/chemistry/sub_unsub.shtmlhttp-:-//www.ccl.net/spammers.txt