Subject: Description of the np* excited state with unrestricted methods

From: "Marta For�s i Bocsh" <[email-masked]>
Date: Tue, 29 Dec 1998 17:11:15 +0100

Dear all,

We are studying the first singlet excited state np* of
1-amino-3-propenal, which results of transferring an electron from the
highest sigma occupied orbital to the lowest p virtual orbital. The UHF
and UDFT methods can not describe this state well because these methods
are based on a single determinant, which is a mixture of a singlet and a
triplet state. The ROHF method could be a good solution to solve this
problem. I would like to know if it is possible to use ROHF in Gaussian
to study the 1np* state of 1-amino-3-propenal (the first singlet excited
state of different symmetry from that of the ground state)? Others
approximations to remedy the spin contamination problem could be the
spin projection of the specific method or the “Sum method”. Does anybody
know which can be the best approach or have some references about this
question?

The UMP2, UQCISD and UCCSD(T) wave functions are not described by only a
single determinant, however, they are described assuming that the
zeroth-order wave function is a single determinant. Can one consider in
these methods that although the wave function is not spin-adapted they
describe correctly the singlet open-shell np excited state of
1-amino-3-propenal?

According to the Gaussian User’s Reference, with the “qcisd=s2t” keyword
one can get the S2 value for this method, however, when I try to do it I
obtain the following error:

QPERR --- A SYNTAX ERROR WAS DETECTED IN THE INPUT LINE.
 #QCISD=S2T
        '
 Last state="PRC1"
 TCursr= 4980 LCursr=   7
 Error termination via Lnk1e in /soft/g94/l1.exe.

Does anybody know which is the mistake I do?

Thanks in advance.

Marta