Subject: Orbital energies in AMPAC

From: Mr Andrew D Allen <[email-masked]>
Date: Mon, 24 Jan 94 10:30:27 GMT

Dear All       
	I am trying to calculate accurate orbital energies for a series of
molecules using AMPAC2.1 under AMSOL. At the moment I am trying two possible
way of obtaining the first excited state orbital, ie the LUMO. 

	a) To use the keyword EXCITED (eqivalent to OPEN(2,2), SINGLET, ROOT=2)
and then record the HOMO energy reported, which I think should be equivalent to
the HOMO of the ground state. This should allow for any geometry changes that
occur during excitation.

	b) To calculate a very accurate ground state an to take the differences
in the eigenvalues reported, for the two middle orbital energies.

The problems I have run into are simply that the EXCITED state calculations are
difficult to converge, and I have initial gradients in excess of 100Kcal/Ang.
The second method is easy to converge, I get some nice Gradients, but the
differences in the eigenvalues for the HOMO and LUMO given seem to be much too
large.

Eg. Benzene     
	Ground State Calculation (Ionisation potential = 9.652 eV)
	Excited State Calculation (Ionisation potential = 3.275 eV)
	Eigenvalues for Groundstate (-9.652 and 0.554 eV)

method a) gives the UV transition as 194.4 nm, method b) gives the UV transition
as 121.5 nm. The experimental is 203.5 nm.

	Can someone give me pointers to the better method of calculating these
orbital energies. I would exclusively use method b) as it it gives results
closer to the experimental, but as mentioned, the excited states are difficult
to converge, and very time consuming to calculate.

Yours in hope Andy.

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Andrew David Allen BSc. Hons (SUR) GRSC.       [email-masked] - Internet.
Structural and Computation Chemistry Group,    [email-masked] - JANET.
Department of Chemistry,                       phone +44(483)-300800-2632.
University of Surrey,                          fax   +44(483)-300803.
Guildford, Surrey, GU2 5XH, UK.                fpt 131.227.110.2
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