*From*: "Kalju Kahn" <kalju .. chem.ucsb.edu>*Subject*: CCL:G: anharmonic vibrational frequencies and IR intensities*Date*: Tue, 18 Oct 2011 21:38:32 -0700

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