CCL: excited states of polyene cations

 Sent to CCL by: makowskm!^!chemia.uj.edu.pl
 Dear Ulrike,
 TDDFT usually gives wrong description of relative enegies of A_g and B_u
 states of neutral polyenes. There was a paper by Head-Gordon showing that
 Tamm-Dancoff approximation to TDDFT works better in this case than popular
 RPA, but still this does not seem to be a method of choice for these
 systems, probably. CASSCF worked quite fine as long as large active space
 can be used. I remember one paper pointing that you need sth like
 double-zeta active space (for octatetraene it would be 4 occupied, 8
 virtual) to obtain very good accuracy. I can search for reference if you
 are interested. It was so as I've checked up to C10. I was testing some
 time ago CIS(D) and SAC-CI but they still had big problems with A_g state
 (the one dominated by double excitation). The similar problems might
 happen for cations - this is out of my knowledge (I did not make any
 calculations on them).
 Yours,
 Marcin Makowski
 --
 Dr Marcin Makowski
 Jagiellonian University
 Department of Theoretical Chemistry
 email:makowskm^-^chemia.uj.edu.pl
 or
 Kyushu University
 email:marcin^-^cube.kyushu-u.ac.jp