Subject: DFT geometry oscillating non-convergence

From: "Bruce. Tattershall" <[email-masked]>
Date: Fri, 1 Sep 2000 19:50:23 +0100 (GMT)

I am calculating geometries for fairly large (7 P,S; 32 C,N; 24 H; no
symmetry) polycyclic molecules, with a view to calculating phosphorus
NMR shieldings.  Using g94, the geometry optimisations eventually
converged at the RHF/3-21G* level, and RHF/GIAO calculations using
these geometries are fairly encouraging in respect of fitting
differences in observed chemical shifts.  Conformations seem to be
controlled by non-bonded interactions between hydrogens on saturated
carbons of 1,2,3,4-tetrahydroisoquinolide substituents, and the
phosphorus sulfide cage.

To do better than this, I need better geometries.  In g98 I am now
trying MPWPW91 or MPW1PW91 DFT optimisations, using Ahlrichs SV basis
with single d polarisation functions on N, P and S, but not on C or H.

(I am using the MPWPW91 rather than BLYP functionals because work elsewhere
seems to show that they give better geometries for heavy main-groups
polcyclics.  The Ahlrichs SV basis works well for RHF geometries on 
polyphosphorus polycyclics, and can readily be followed by his TZV basis
for sp calculations.)

Using either DFT method, starting from the RHF/3-21G* geometry, the
geometry fails to converge to the default g98 limits.  Energy and
forces go down nicely for the first twelve geometry points, but before
that, when the forces are already well within the g98 convergence
limits, the geometry starts making large displacement jumps.  After 16
points, energy stops converging and starts to oscillate, along with
forces and displacements.  The optimisation seems to be in a bumpy
very flat-bottomed minimum which g98 seems unable to settle into the
bottom of.  Visually (using molden) one can correspondingly see the
geometry oscillating between what look like fairly well repeated
conformations.  The coordinates which give the big displacements are
dihedral angles defining the conformation of the saturated part of the
isoquinolide bicycle.  The tendency of this to settle to a particular
conformation seems to be interfered with by its non-bonded
interactions with the PS cage.  At the MPWPW91 level, I tried starting
a fresh opt run, inputting coordinates from one of the best points so
far, to see whether g98 could be jogged into setting off in a slightly
more successful direction, but this had a similar result.  I know
I can use opt=loose in g98, but will this help me to get a good enough
geometry?

Please can someone advise?  I am sorry if this is all old news in CCL,
but as a newcomer I failed to find anything clearly useful in several
hours of dredging through the archive using the search terms I thought
of.

Many thanks,   
Bruce Tattershall
Department of Chemistry, Univ. of Newcastle, England.
email:  [email-masked]