From owner-chemistry@ccl.net Wed Oct 19 08:19:01 2011
From: "Kalju Kahn kalju[A]chem.ucsb.edu"
To: CCL
Subject: CCL:G: anharmonic vibrational frequencies and IR intensities
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X-Original-From: "Kalju Kahn"
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Date: Tue, 18 Oct 2011 21:38:32 -0700
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Sent to CCL by: "Kalju Kahn" [kalju!A!chem.ucsb.edu]
Dear Steve,
The answer partially depends on the QM model; oftentimes anharmonic
frequencies are provided for methods for which analytical second
derivatives are available. For example, Gaussian supports anharmonic
fundamentals at HF, DFT, and MP2 level ... but not at the CCSD level.
I believe that most program that allow for anharmonic fundamental
frequencies should be able to produce overtone and combination frequencies
as well. It may be not on by default; in MOLPRO, set COMBI=1; in CFOUR,
request ANHARM=FULLQUARTIC.
NWChem's VSCF module can be used to calculate anharmonic frequencies at
any level of theory using numeric grid, but dipole derivatives are
available only at the SCF level.
A standalone program SPECTRO allows calculation of anharmonic infrared
spectra from energy and dipole moment derivatives. People sometimes write
their own codes to calculate anharmonic frequencies and intensities based
on ab initio energies. For example, JCP recently published our
convergence study on anharmonic IR intensities in hydrogen fluoride (J.
Chem. Phys. 135, 131103 (2011)). The Mathematica code that evaluates
fundamental and overtone intensities from the anharmonic force field and
dipole function is available upon request ... just contact me directly.
Hope this helps,
Kalju
> Sent to CCL by: Steve Williams [willsd]^[APPSTATE.EDU]
> I am interested in methods for ab initio computation of IR spectra and
> am curious about codes that can perform some or all of the steps below.
> I know about scale factors for frequencies and force constants to
> improve harmonic frequencies, and this is not what I'm interested in
> here. I'd like to find out about what ab initio codes can:
> 1) Compute anharmonic frequencies for fundamental modes,
> 2) Compute anharmonic frequencies for overtones and combination bands,
> 3) Compute anharmonic IR intensities for fundamental modes,
> and 4) Compute anharmonic IR intensities for overtones and combination
> bands.
>
> I know the following: Gaussian and Qchem can do 1 and 2, but not the
> intensities in 3 and 4.
> GAMESS-US and Cfour can do 1,2,3, and 4.
> Molpro can do 1 and possibly 2, but I'm not sure about 3 or 4 (I don't
> use molpro and I did find in the online manual explict statements about
> overtones, combination bands, and IR intensities.)
>
> I'd appreciate any information about any ab initio codes with
> capabilities in any of these areas.
>
> Thanks,
> Steve Williams>
>
>
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Kalju Kahn
Department of Chemistry and Biochemistry
UC Santa Barbara, CA 93106