CCL: Predicting the Electronic Spectrum of 1,3-butadiene

 Sent to CCL by: [eurisco1(-)pochta.ru]
 Dear Gustavo L.C. Moura,
 
 Is the 1,3-butadiene a known problematic molecule for spectroscopy?
Do you mean for experimental measurements? In general, if high precision and accuracy are required, any system can be considered such a difficult case.
How can both my calculation and the best >theoretical estimate be so far from experiment?
Yes, it happens. :-) Sometimes it is very difficult to catch correlation effects even if it is so-called "best theoretical estimate". Maybe you need to play a lot with active space and /or expand basis set. I would suggest to go beyond CASPT2 and apply MRCI (there are several proposed methods - not only SORCI and dft-mrci hybrid). 
 
 How can DFT results be >so close to the experimental result?
Yes, it happens. :-) DFT is black box , TD-DFT is black box in black box. Maybe you need to check different functionals.
So far, with the exception of the 1,3-butadiene molecule, all my SORCI calculations are very close to the experimental 
 Really, it is difficult to say. There is a lot of uncertainty.
 Sincerely,
 Ol Ga
-----Исходное сообщение----- 
 From: Gustavo L.C. Moura gustavo.moura%%ufpe.br
 
 Sent: Monday, October 10, 2011 10:55 PM
To: Ga, Ol Subject: CCL: Predicting the Electronic Spectrum of 1,3-butadiene 
 Sent to CCL by: "Gustavo L.C. Moura" [gustavo.moura!A!ufpe.br]
 Dear CCL Readers,
As one step of a larger project, I have calculated the excitation energy for the first singlet excited state of 1,3-butadiene using the SORCI technique with the TZV(2D,2P)++ basis set. The result I got was 6.37eV (~195nm). In the article M. Schreiber, M. R. Silva-Junior, S. P. a Sauer, and W. Thiel, Benchmarks for electronically excited states: CASPT2, CC2, CCSD, and CC3., The Journal of Chemical Physics, vol. 128, no. 13, p. 134110, Apr. 2008, it is quoted that the best theoretical estimate from ab initio calculation for this excitation is 6.18eV. Therefore, my calculated result is reasonably close to the best theoretical estimate. However, the experimental gas phase value for this transition is 217nm (~5.7eV), as can be seen from 
 http://www.atmosphere.mpg.de/spektrum_image3.php?subkat=Dienes&kat=Alkenes%2Cdienes%2Bradicals&file=CH2%3DCHCH%3DCH2_218-333K_lin.JPG
Therefore, both my result and the best ab initio estimate are not close to the experimental result. In the article M. R. Silva-Junior, M. Schreiber, S. P. a Sauer, and W. Thiel, Benchmarks for electronically excited states: time-dependent density functional theory and density functional theory based multireference configuration interaction., The Journal of Chemical Physics, vol. 129, no. 10, p. 104103, Sep. 2008, it is quoted a value of 5.74eV from B3LYP calculations for the first singlet excited state of 1,3-butadiene.
Is the 1,3-butadiene a known problematic molecule for spectroscopy? How can both my calculation and the best theoretical estimate be so far from experiment? How can DFT results be so close to the experimental result? So far, with the exception of the 1,3-butadiene molecule, all my SORCI calculations are very close to the experimental ones. 
 Sincerely yours,
 Gustavo L.C. Mourahttp://www.ccl.net/cgi-bin/ccl/send_ccl_messagehttp-:-//www.ccl.net/chemistry/sub_unsub.shtmlhttp-:-//www.ccl.net/spammers.txt