I am trying to compute UV/Vis absorption spectrum for Ethene using Gaussian 09.

I created the ethene molecule in GaussView 5, and cleaned it. Then I optimized the molecule and checked if I got the minimum energy conformation with the following:

# freq=noraman cphf=noread b3lyp/6-31g(d) geom=connectivity formcheck 
 integral=grid=ultrafine scf=maxcycle=1000

Next I performed the energy calculation as follows:

# td b3lyp/6-31g(d) geom=connectivity

As a result I got a single peak with the maximum at 134 nm (see picture below) which seems to be way off from what I googled around on the Internet, which is like 173 nm or 180 nm. enter image description here

Can anyone please help me figure out if I have done it correctly and whether I am way off the experimental data?


In short, there are two obvious problems with the setup OP uses for TD-DFT calculations:

  1. B3LYP functional is not a good choice for TD-DFT.
  2. 6-31G(d) basis is usually too small.

At M06-2X/Def2-TZVP level I get a maximum at ~160 nm, which, taking the accuracy of the TD-DFT approximations into account, is close enough to the experimental value.

  • 1
    $\begingroup$ Thank you @Wildcat. I tried to redo the calculation according to your suggestion. I just took the molecule optimized at b3lyp/6-31g(d) and calculated the energy as follows: # td M062X/Def2TZVP geom=connectivity. I got the maximum 158.53 nm, so close to what you kindly reported. I wonder however if it's OK what I did in terms of optimizing the geometry at a different level than the one used to do the TD calculation for UV/Vis spectra? Looks like it works here. Do you think I do not need to reoptimize all of my molecules then? $\endgroup$ Sep 15 '16 at 1:14
  • $\begingroup$ B3LYP/6-31G* might be a bad choice for a lot of calculations but it is not to be doomed a priori. We calculate a lot with it and reach acceptable accuracy. It always depends on the molecule that you are investigating. M06-2X for example does not give acceptable excitation energies for my complexes. Not even with triple zeta bs while B3LYP/6-31G* is way better for us. $\endgroup$ Sep 15 '16 at 8:00
  • $\begingroup$ B3LYP/6-31G* might occasionally spit out some good numbers for excitation energies, right, but I would not trust this level of theory anyway since even when it works fine, it is very likely just the result of a fortunate cancellation of errors. $\endgroup$
    – Wildcat
    Sep 15 '16 at 9:01
  • $\begingroup$ @RicardoMoreno, B3LYP/6-31G* geometry of ethene should be good enough, so that there is no need for reoptimization. $\endgroup$
    – Wildcat
    Sep 15 '16 at 9:03
  • 2
    $\begingroup$ @pentavalentcarbon, why do you say that "M06-2X is not meant to be used for excitation energies"? It is one of the best hybrids according to benchmarks (10.1002/qua.24438). $\endgroup$
    – Wildcat
    Oct 1 '16 at 22:56

For smaller molecules like ethene, the webbook by NIST is a good source of experimental data. Search results for ethene in particular here yield both a figure of a vacuum UV spectrum, indicating an absorption maximum centred around 160-165 nm with $\log \epsilon$ of about 4.2, as well with a primary literature reference (Platt, J.R.; Price, W.C., J. Chem. Phys., 1949, 17, 466).
Of course, other databases like Scifinder/Chemical Abstracts by the American Chemical Society may indidicate more recent data. A specialized compilation, more suitable for ethene in particular, is the MPI-Mainz UV/VIS Spectral Atlas of Gaseous Molecules of Atmospheric Interest here, with an entry about ethene.

Side note: NIST database equally offers access to other thermophysical data (like Vibrational and/or electronic energy levels) potentially of interest for you, too.

Addendum: With the searchable Computational Chemistry Comparison and Benchmark DataBase, NIST equally hosts numerous results of calculations of energies and structures for small molecules. This that may serve as a valuable reference, comparing the methods with each other, as well against experimentally determined data.

  • $\begingroup$ Dear @Buttonwood, thank you so much for your guidance and the awesome links! By the way, do you think that the # td M062X/Def2TZVP level as suggested by Wildcat (and it seems to be working fine for ethene) will be good enough for UV/Vis computation for larger molecules, let's say 20-25 heavy atoms like carbon, and only a few oxygens or nitrogens around? $\endgroup$ Sep 15 '16 at 11:52
  • $\begingroup$ @Ricardo Moreno You are wellcome. Yet: My practice in computational chemistry is too limited to provide you sound advice about the method / basis set to be deployed other than too look out how similar questions were solved by others in the field. Drawing a structure, (pre)optimising here and then with a force field & PM3 as a starting point / comparision e. g. for powder diffraction differs on what your question adresses and (in terms of computational cost) is at least a league below b3lyp and other hybrid functionals. $\endgroup$
    – Buttonwood
    Sep 15 '16 at 14:46
  • 1
    $\begingroup$ @RicardoMoreno Take a tour d'horizon in CCBDB just discovered. (Ethene is present, too.) $\endgroup$
    – Buttonwood
    Sep 16 '16 at 17:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.