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I watched a ted talk (Luca Turin: The science of scent) and was wondering if anyone had any examples of how to calculate vibrational spectra of molecules (used as fragrances)? Would love to figure out how he computed these charts.

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  • $\begingroup$ Going off this definition: A molecule is formed when two or more atoms join together chemically. A compound is a molecule that contains at least two different elements. All compounds are molecules but not all molecules are compounds. I would like to start with the easiest which looks like molecules and go towards compounds. $\endgroup$ – Rick T Apr 9 '15 at 10:52
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The usual approach is:

  • Find the equilibrium geometry (within the Born-Oppenheimer app.).
  • Expand the energy in Taylor series till second order term. The first order will be 0 because of the minimum energy condition.
  • Include this expression in the Hamiltonian.
  • Generate a coordinate transformation to get normal coordinates.
  • Relate second order derivative of the potential energy with the force constant of Hook law (Using second order term we can identify it with a harmonic oscillator). Then get the harmonic frequencies.
  • The software employed should give also the intensity for each frequency.
  • Plot intensity vs. frequency/energy/wave length. You'll get only "points". To transform them to a continuous function you can use a Lorentzian like function.

Details can be found in most computational quantum chemistry books.

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You can build molecules and calculate vibrational frequencies with Molcalc. Molcalc does have a molecular size limit, so for bigger molecules you must download some programs. I have written about computing vibrations with the GAMESS program.

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