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I already did an IR frequency calculation for my molecule. Is there any possibility to just run a Raman frequency calculation with the chk file from my IR calculation?

Or do I have to do the calculation again with the freq=raman keyword? I checked the Gaussian manual and found the raman=polar keyword, but I think this is not exactly what I am looking for.

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    $\begingroup$ Related (unanswered) question on researchgate. The paper mentioned suggests it was possible in Gaussian09, so there should be a keyword for this. A combination of freq=SaveNormalModes in the frequency calculation and freq=ReadNormalModes in the Raman calculation may do the trick, but I haven't gotten a chance to test it. $\endgroup$ – Tyberius Dec 2 '20 at 18:19
  • $\begingroup$ Thanks, I will try it today and will report later how it worked. $\endgroup$ – Andrea Dec 3 '20 at 6:16
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    $\begingroup$ @Tyberius @Andrea I tried it with the keywords freq=SaveNormalModesand freq=ReadNormalModes. I used Methanol as an easy example. First I run an optimization followed by a normal frequency calculation by using the geom=allcheck and the suggested freq=SaveNormalModes keywords. The last calculation a raman with this Routesection %chk=methanol_raman.chk %oldchk=methanol_freq.chk #p GFINPUT POP(FULL,NBO) B3LYP/6-31G(d,p) freq=(raman,ReadNormalModes) geom=check The calculation ends with the error FileIO operation on non-existent file.. $\endgroup$ – Till Dec 4 '20 at 12:30
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Raman intensities require polarizabilities to be calculated, IR intensities only dipole moments. Your checkpoint file may therefore not have the properties you require, in which case you'll need to do more calculations. Using the polar=raman keyword would apparently save you a little bit of time, because the program then already knows your force constants and normal modes from the preceding freq calculation, but there will still be computational effort involved in calculating the polarizabilities.

Note that there are some cases where Gaussian also give you the polarizabilities "for free", as the manual states on the polar=raman page:

Note that Polar is done by default when second derivatives are computed analytically.

You can use the chkchk tool to see what's really stored in the checkpoint file.

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