Subject: wavefunction instability vs. spin contamination

From: Theo de Bruin <[email-masked]>
Date: Thu, 31 Aug 2000 11:46:23 +0000

Dear CCL'ers

At the moment we are performing calculations on several (relatively large)
di-cobalt complexes using the B3LYP functional and LanL2DZ basis set.
Restricted B3LYP calculations yield geometries that fit quite well the
corresponding crystal structures.  However, from computations with the G98
keyword STABLE it was clear that wavefunctions showed a RHF --> UHF
instability. For example:  Eigenvector   1:   Triplet-?Sym  Eigenvalue =
-0.0067007  (it seems that the negative eigenvalue is rather small....?)
Performing the same calculations with the UNrestricted B3LYP (using
guess=mix and changing the DIIS for the QC algorithm) does yield stable
wavefunctions, which are about 0.1 to 0.2 kcal/mol lower in energy.
However, the obtained geometries with the unrestricted wavefunctions
deviate substantially from the crystal structures (delta r(Co-Co) > 0.2
angstrom!), and moreover there is a large spin contamination (S**2 is up to
0.74 for a singlet ground state), which probably results from the (too)
large Co-Co distance.

We have calculated that the triplet electron configuration has a much
higher energy (> 50 kcal/mol).  Unfortunately, we were not able to perform
successful CASSCF calculations to find out if there was contribution of the
triplet in the ground state, since the necessary number of electrons and
orbitals in the active space was far too large.

Since MP2 calculations on 1st row transition metals are unreliable, higher
(than HF or DFT) correlations methods are completely impractical for this
size of systems, and the instability seems to be very small (small neg.
eigenvalue and small energy difference between restr. and unrestr.
wavefunctions), it is very tempting to continue the work with the
restricted B3LYP method.  Especially, because we are mostly interested in
the geometries and their relative energies.

Does anybody have some tips or advice how to continue?

Best regards,
Theo de Bruin

-----------------------------------------------------------------------
Theodorus J.M. de Bruin           Phone  +33 4 76 63 56 28
Université Joseph Fourier         Fax    +33 4 76 51 43 82
LEDSS VII  Chimie Théorique       E-mail [email-masked]
UMR CNRS 5616
Bâtiment 52 - Chimie Recherche
DU BP 53
38041 Grenoble Cedex 9
FRANCE
-----------------------------------------------------------------------