Hi,
I'm running Gaussian
16. Could
someone please help me understand how "number of steps" and MaxCycles
work in Gaussian? I have a molecule whose geometry I want to optimize
to a local minimum, at the B3LYP/6-31G(d,p) level. I tried this method
section in my input:
# Opt(MaxCycles=300)
B3LYP/6-31G(d,p)
In the documentation for the Opt
keyword (
https://gaussian.com/opt/?tabid=1#Opt_keyword__MaxCycles_option
), it says that "MaxCycles=N sets the maximum number of optimization
steps to N. The default is the maximum of 20 and twice the number of
redundant internal coordinates in use (for the default procedure) or
twice the number of variables to be optimized (for other
procedures)."
I'm
not sure what it means by "redundant internal coordinates." I
did not
explicitly specify redundant coordinates (as I think one might do for a
Scan calculation). My Z-matrix has 31 lines (31 atoms). On the other
hand, my variables section contains 87 lines: 30 bonds (B1-B30), 29 bond
angles (A1-A29), and 28 dihedral angles (D1-D28).
When
I run my optimization, Gaussian terminates after a few hours with a
Link 9999 error; the end of the log file reads, "Error termination
request processed by link 9999." I think this usually means the
optimization has failed to converge, and if I scroll up a bit in the log
file, I see that is the case. In the final "GradGrad..."
section, it
reads:
Optimization stopped.
-- Number of
steps exceeded, NStep= 186
-- Flag reset to prevent
archiving.
Why
specifically did optimization stop after 186 steps -- what am I missing
here? Are "number of steps" and MaxCycles the same? If
so, why didn't
optimization continue until NStep=300 was reached or until the
calculation converged? If not, how are they
different?
I
was able to reach convergence by running a new optimization -- with the
last geometry of the previous run as the starting configuration -- but
in the future, how can I increase the maximum number of allowed
steps?
(By
the way, if I remember correctly, the default number of cycles is 128
if MaxCycles is not specified. Also, I believe the notation used in
older versions of Gaussian was, for example, 'OptCyc=300' but I believe
it's now 'Opt(MaxCyc=300)' for Gaussian 16. See, for example,
https://gaussian.com/obsolete/ I think that 'MaxCyc' is just
an allowed abbreviation of
'MaxCycles'.)
Thank you very much for your
time!
Best,
Andrew
Andrew
DeYoung, PhD
Department of Chemistry
Carnegie Mellon
University