Subject: CCL: G92: SCF Convergence Problem: 6-31+G*

From: "Ben Burke" <[email-masked]>
Date: Thu, 3 Jul 1997 10:44:32 -0700

Dear G92-ers:
  I was unable to optimize two systems when I changed from the 6-31g* basis
set to 6-31+g* basis set.  The systems of interest are neutral and anionic
heterocycles, containing C,O,N,H.  I optimized the neutral species at the
6-31g* level (no +), and obtained the final geometry.  I used this geometry
(without a polar proton) as the starting point for the optimization of the
anionic species.  The SCF calculation for the anionic species would not
converge under RHF or UHF conditions, even using the quadratic convergence
option.  I checked the wavefunction of the step prior to this one, and the
wavefunction is stable.
  After these investigations with the anionic species, I tried to optimize the
neutral species at the 6-31+g* level, and it failed.  Has anyone seen anything
like this before (something converges at 6-31G*, but fails at 6-31+G*)?
I used G92 for the calculations, running on an SGI R8000, IRIX Release 6.1.
A few details on the calculations are given below after a longer description.
Any ideas what to try next?  Of course, I will pass along a summary of the
responses.
Thanks,
--Ben

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Long version with a few details folowing:
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I optimized a neutral heterocyclic structure at the 6-31g* level.

(neutral)  rhf/6-31g*               E(RHF) =   -680.159849444 A.U.

I was interested in the properties of the anion (charge=-1).  So, starting with
the 6-31G* optimized structure, I removed an NH proton, and submitted a job
for optimization at the 6-31+G* level.

In the second step of the minimization, the SCF fails to converge.  I tried
to remediate (or evaluate) in several ways:
1.  I repeated the run with scf=(novaracc) to maintain integral accuracy, but
the job failed exactly as before.
2.  I investigated the stability of the wavefunction in the prior step, and
it showed no instabilities (see anion, run 2).
3.  I tried the optimization with scf=qc, and a large scfcyc value in order
to try to get convergence  (see anion, run 3).
4.  Although the energy separation seems adequate to make sure that there
is spin-pairing, I ran the job using UHF in order to make sure that some
di-radical is not be preferred.  So far, there is no convergence after 1001
steps.

When reviewing what I had done, I decided for completeness sake that I should
try to optimize the neutral species using diffuse functions (6-31+g*).  It
failed!

(1)  rhf/6-31+g*               E(RHF) =  -680.134141544 A.U.
                               E(RHF) =  -686.040030921 A.U. --> Fails

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A few details on the submissions and consequent output
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******** (anion, run 1a: Failed Run) ********
 rhf/6-31+g* scf=direct opt test pop=full

 SCF Done:  E(RHF) =  -679.621506401     A.U. after   22 cycles
             Convg  =    0.8071D-08             -V/T =  2.0031
             S**2   =   0.0000
 Copying SCF densities to generalized density rwf, ISCF=0 IROHF=0.
 ....
 >>>>>>>>>> Convergence criterion not met.
 SCF Done:  E(RHF) =  -684.524654296     A.U. after   65 cycles
             Convg  =    0.8578D-03             -V/T =  2.0078
             S**2   =   0.0000
 Convergence failure -- run terminated.

******** (anion, run 2: Check Stability of Prior Step) ********
 rhf/6-31+g* scf=(direct,novaracc) stable=opt test pop=none geom=check

 The wavefunction is stable under the perturbations considered.
 The wavefunction is already stable.
 E(RHF) =  -679.621506401

******** (anion, run 3: Optimize with Quadratic Convergence) ********
 rhf/6-31+g* scf=(direct,novaracc,qc) opt scfcyc=1000 test pop=reg geom=check

 Iteration  96...
 The polynomial fit failed.  Using point  1.
 A contracting polynomial of degree 16 produced  0.0000
 Search did not lower the energy significantly.
 No lower point found -- run aborted.

******** (anion, run 4: Try with UHF) ********
 uhf/6-31+g* scf=(direct,novaracc) opt scfcyc=1000 test pop=reg geom=check

 SCF Done:  E(UHF) =  -679.621506401     A.U. after   22 cycles
 >>>>>>>>>> Convergence criterion not met.
 SCF Done:  E(UHF) =  -684.550622760     A.U. after 1001 cycles
             Convg  =    0.4300D+01             -V/T =  2.0078
             S**2   =   0.0000

-- 

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Benjamin J. Burke        Agouron Pharmaceuticals   [email-masked]
Research Scientist       3565 General Atomics Ct   Tel: (619) 622-3018
Computational Chemistry  San Diego, CA     92121   FAX: (619) 622-3299
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