Subject: G94: Problem with PES Scan in DFT

From: "M. Nicklaus" <[email-masked]>
Date: Wed, 29 Jan 1997 11:06:38 -0500 (EST)

Dear CCL'ers,

I seem to be having a problem with a Potential Energy Surface (PES) scan
in G94.  I am running a relaxed PES scan on a nucleoside analog, trying
to rotate the base.  I am running this job at the DFT method/basis set
B3LYP/6-31G(d).  I'm using cartesian coordinates (a newzmat-generated
input file with a Z-matrix didn't work previously), pre-optimized at
the same level of theory from X-ray coordinates.

For the first few conformations, everything seems to have proceeded nicely;
i.e., the energy started out some 20 kcal/mol or so above the (presumed)
global energy minimum, and then came down, during the partial optimization,
to a reasonable value a few kcal/mol above the global min.

With the current conformation, however, the job seems to be stuck, since
for nearly 20 steps now, the energy has only been oscillating in a narrow
band.  Here's a grep of the output file (slightly modified through a filter
--don't worry about the origin of the kcal/mol scale):
 SCF Done:  E(RB+HF-LYP) =     6.587  kcal/mol (  -877.288353458 A.U.)  after   12 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.501  kcal/mol (  -877.288490683 A.U.)  after   12 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.470  kcal/mol (  -877.288539921 A.U.)  after   11 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.443  kcal/mol (  -877.288582731 A.U.)  after   12 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.438  kcal/mol (  -877.288591742 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.439  kcal/mol (  -877.288590115 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.438  kcal/mol (  -877.288591793 A.U.)  after    9 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.438  kcal/mol (  -877.288591832 A.U.)  after    8 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.440  kcal/mol (  -877.288588866 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.438  kcal/mol (  -877.288591957 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.439  kcal/mol (  -877.288590211 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.438  kcal/mol (  -877.288592040 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.440  kcal/mol (  -877.288588983 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.438  kcal/mol (  -877.288591984 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.440  kcal/mol (  -877.288588800 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.438  kcal/mol (  -877.288591901 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.439  kcal/mol (  -877.288589705 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.438  kcal/mol (  -877.288592037 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.440  kcal/mol (  -877.288588677 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.438  kcal/mol (  -877.288591887 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.439  kcal/mol (  -877.288589980 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.438  kcal/mol (  -877.288592074 A.U.)  after   10 cycles           
 SCF Done:  E(RB+HF-LYP) =     6.440  kcal/mol (  -877.288588420 A.U.)  after   10 cycles           

I'd hate to kill this job--which has already taken up ca. 20 CPU-days--if there'e any
chance that it's going to converge at some reasonable time.

So here are my questions: 
(a) Can anyone give me a hint as to what's going on here?  Has anyone seen
    a similar behavior before?

(b) Should I kill this job now, or wait a bit longer?  How much longer?

(c) If this is a known effect, is there anything I can do to avoid it?

Marc

------------------------------------------------------------------------
 Marc C. Nicklaus                        Lab. of Medicinal Chemistry
 e-mail: [email-masked]               National Cancer Institute, NIH
 Phone:  (301) 402-3111                  Bldg 37, Rm 5B29
 Fax:    (301) 496-5839                  BETHESDA, MD 20892-4255    USA
         WWW:  http://www.nci.nih.gov/intra/lmch/MCNBIO.HTM
------------------------------------------------------------------------