Subject: CCL:How to calculate excitation energy in high accuracy by

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Mon, 2 Oct 2000 11:29:59 +0100

Kiniu WONG Kin-Yiu:

>     I couldn't get an accurate excitation energy by Gaussian, even a simple
> atom such as Hydrogen and Beryllium. The excitation energy of H is 10.2eV, but
> by UCIS(50-50)/6-31+G(d), it gives the excitation energy = 26.1eV:
> 
>  CIS wavefunction symmetry could not be determined.
>  Excited State   1:   ?Spin  -?Sym   26.1001 eV   47.50 nm  f=0.0000

Dear Kiniu,

calculation of excitation energies of the H atom with the 6-31+G(d) basis set 
is meaningless.  The electronic transitions of atomic hydrogen are of 
Rydberg-type, corresponding to a change in the main quantum number n: 1s->2s, 
1s->2p, and so forth.  But the basis set 6-31+G(d) does not include any 
"diffuse functions" on hydrogen, and is unsuitable for description of excited 
states.  Inclusion of a 2s-type diffuse function leads to a drastic decrease in 
the computed transition energy:

6-31++G(d):   1s->2s = 10.4 eV

However, this basis set does not predict the three optically allowed 1s->2p 
transitions, that are energetically degenerate with the optically forbidden 
1s->2s transition.  Computation of 1s->2p transitions requires basis sets with 
an accurate representation of 2p functions. But degeneracy with 1s->2s is not 
easily predicted with standard sets, for example:

AUG-cc-pVTZ:  1s->2s = 10.2 eV, 1s->2p = 11.2 eV
AUG-cc-pVQZ:  1s->2s = 10.2 eV, 1s->2p = 10.9 eV
AUG-cc-pV5Z:  1s->2s = 10.2 eV, 1s->2p = 10.7 eV

Evidently, different angular momenta are treated at different levels of 
accuracy.

Yours, Jens >--< 

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JENS SPANGET-LARSEN         Phone:  +45 4674 2000  (RUC)
Department of Chemistry             +45 4674 2710  (direct)
Roskilde University (RUC)   Fax:    +45 4674 3011 
P.O.Box 260                 E-Mail: [email-masked]
DK-4000 Roskilde, Denmark   http://www.rub.ruc.dk/dis/chem/psos/
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