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## Plotting Stick Spectra For Vibrational Frequency CalculationsAfter a calculation of the vibrational frequencies, you will find the
file irspectrum_harmonic.txt in the working directory; if you have done a VPT2
calculation, you will also find a file called irspectrum.txt. Both of these
offer a visual representation of the calculated infrared spectrum, and can be
plotted with gnuplot as follows: which of course can be greatly elaborated upon (and combined with laboratory spectra) if you have a working knowledge of the gnuplot plotting package that is available with essentially all installations of unix/linux/etc. The filenames are self-explanatory: the irspectrum_harmonic.txt contains the harmonic
frequencies for the fundamental vibrations and the associated intensities (in wavenumbers
and km/mol, respectively), while irspectrum.txt contains the corresponding numbers from
VPT2 calculations. In the latter case, when strong resonances (closely-lying levels differing in an odd number of excitation (Fermi resonances), quite peculiar-looking
spectra can
be generated; use of the For irspectrum.txt, the fundamental and two-quantum transition intensities are written to the
second and third records on each line, respectively. Hence, to differentiate these, you can use the gnuplot command The VPT2 analysis in CFOUR is limited to one- and two-quantum levels for intensities; hence
using this graphical display of data will reveal nothing about higher-quantum levels. Again, Finally, for the VPT2 output, and for gnuplot aficionados, there is a set of "labels" for each of the peaks in
the stick spectrum. To see these (they are stored in the file irspectrum.labels), just invoke the command |

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CFOUR is partially supported by the U.S. National Science Foundation.