CCL:G: SCF convergence failure with AUG-cc-pVDZ basis in Gaussian 09

["Douglas B. Staple" <dstaple###dal.ca>]

 Sent to CCL by: "Douglas B. Staple" [dstaple- -dal.ca]
 I received several good suggestions from various people:
 1) S. Lehtola suggested deleting the diffuse functions from the hydrogens.
 2) S. Lehtola also suggested SCF=XQC, and trying Jensen's basis sets.
 3) K. Peterson suggested removing the diffuse s function for each atom.
 4) H. A. J. Vazquez suggested SCF(NoVarAcc,NoIncFock,TIGHT)
 5) V. Chaban suggested decreasing SCF requirements, e.g. SCF=Conver=7
I should say right away that the calculation converged fine with
 SCF(NoVarAcc,NoIncFock,Tight). To me, this is a bit of black magic.
 From
 reading the Gaussian 09 manual, I thought that NoIncFock and Tight were
 the defaults, but deleting them causes convergence to fail. So I'm not
 sure yet which of these keywords are necessary for convergence or why.
 Nevertheless, thank you very much for getting my calculation to
 converge!
Reducing the number of diffuse functions as suggested by S. Lehtola
 and
 K. Peterson seems like a very logical and good idea. Unfortunately the
 version of Gaussian 09 I have (Revision C.01) doesn't have the calendar
 basis functions, and I honestly don't know how to reduce the number of
 diffuse functions in AUG-cc-pVDZ any other way.  I'm looking into
 getting access to g09 Rev. D, which has the calendar basis sets.
 The job with SCF=XQC failed to converge; the error message was:
  Linear equations converged to 1.646D-10 1.646D-09 after    31 iterations.
  Density matrix breaks symmetry, PCut= 1.47D-04
  Rerun with SCF=IntRep.
I didn't try rerunning with SCF(XQC,IntRep), because I'd previously
 tried the QC-version of that to no avail.
The job with SCF=Conver=7 failed to converge; the error message was
 "Convergence failure -- run terminated." as before.
 Thank you all very much for your help!
 Doug Staple
 dstaple++dal.ca