Subject: Summary: Gaussian Convergence failure!

From: "William Wei" <[email-masked]>
Date: Wed, 14 May 2003 14:33:24 -0500

Original question:
After several Convergence failure (I changed conformations), what others I
can do for it? My system is about 20 heavy atoms, include one benzal group
in the end, and one Fluorine one Sulfur. After B3LYP/3-21G optimized, it can
not convergence at B3LYP/6-31+G(D) level. Could anybody give me suggestion.
I will summarize the result. Thank you.

I got a lot of messages and I learned a lot from them. It is good to share
these information with our community. Here they are:

---------------------
I suppose, main part od the problem is connected with the presence of
diffuse functions.
Firstly, does your system really need them to be properly discribed?
If so, I would suggest:
1. using UltraFineGrid,
2. maybe using scf=qc, which is slower, but more reliable algorithm,
3. maybe trying to converge it in a small basis containing diffuse
function like 3-21+G, and later using these results as a guess in
subsequent computations.

Yours,

Marcin Makowski
----------------------
Are you using the checkfile from the successful opt
as a start for the 6-31+G(d) calc. perhaps you'd be better to opt at 6-31
first, then do for the bigger basis set.

Regards

Roma
--------------------
Go at it slowly. Do a guess=read to try B3lYP/6-31G, then 6-31G(D) and
then 6-31+G(D). I find this often works. I assume you are talking about
SCF convergeence.

Brian.
----------------------
try to suppress the (+) diffuse function. It worked for me for an organic
molecule, once.
Yours
Alexandre HOCQUET
----------------------
You're not too specific on what you have already tried, so the advice I
will give you is very basic stuff. You might actually need some more
advanced tricks, but you should try these things first. I assume that
you have optimized the geometry in the B3LYP/3-21G* job, and now the SCF
won't converge with the larger basis set? I would at first avoid the
diffuse functions, and instead run a single point job at the
B3LYP/6-31G* level, reading in the initial guess from the checkpoint
file from B3LYP/3-21G* job (Guess=Read keyword). You should also
increase the number of SCF cycles, by specifying SCF(MaxCycle=1000) or
an even higher number if nessecary. Hopefully, this will lead to SCF
convergence. Then you can move on with optimizing the geometry or
running the 6-31+G(D) job, again reading the initial guess from the
checkpoint file. Once you have a wavefunction that converges, you might
also want to run a job specifying the Stable keyword just to make sure.

Hope it helps!

Peter
---------------------
Try scf=qc, it is slower but usually it works when the
default algorithm fail.

Michele Lunelli
--------------------
Most programs will let you use the SCF MOs from one run as the input to
start
the next run.  I would go 3-21 -> 6-31+G -> 6-31+G(D), all in one job.  Are
you
using Gaussian?

John McKelvey
--------------------
I had the same problem earlier.  Check the .out file to see if the failure
is
in the scf calculation.  If so, try using scf=qc in the route section.

Joey
--------------------
Can you try to increase the maxcycle number for scf? For example,
scf=(maxcycle=256) option.

Yubo
--------------------
       Here are some possible approaches to solving your problem.

If it is the '+' that caused the convergence difficulty then try
doing a single point B3LYP/6-31G(d) calculation first.
If the job converged you may then proceed to a 6-31+G(d) calculation with
the guess=read option. Gaussian is capable of taking advantage of
an initial guess generated from a smaller basis set.

   Did you also try any hartree-fock calculation? If the HF method
converged easily then you may try a BHandHLYP singlet point job.
Use the BHandHLYP orbitals which should be
similar to B3LYP as the initial guess  for subsequent calculations.

   Is your B3LYP/6-31+G(d) job a single point energy calculation?
If the convergence failure occurs during
geometry optimization reducing the step size may help.

   In the most difficult case the SCF solution will keep
oscillating. You may then consider measures like reducing
the convergence criteria (with scf=sp; ok for single point
job, not recommended for geometry optimization) or
the scf=qc which could take forever.

Wai-To Chan
--------------------
I assume your convergence failure is not an SCF convergence problem (this is
not clear from your message). Then, if you have done a frequency calculation
for the B3LYP/3-21G optimized structure, try using the force constants for
the calculation at the higher level, i.e. use opt=readfc (note that the name
of the checkpoint file must be the one you used for the lower level
calculation).
Dmitri
--------------------

I am not sure where your problem is, because you did not attach your input
gaussian file with the structure of the molecule.
But, if I have a problem with convergence in Gaussian
(it is true especially for metal complexes)
I usually use the QC option in SCF calculations
So, write "SCF=QC" in your input file.
Unfortunately you will need more time for your calculations :-(

Best regards

Kzys
--------------------
Yes, qc is often needed for such systems (namely S-Fe bonding). One usually
starts with a HF calculation and a small basis, then increases the basis
using options scf(qc,conver=8), then b3lyp calculation (with the same
options)

[email-masked]
----------------------
You might also want to consider reading in the results from the 3-21g
calculation as the guess for the higher basis-set 'guess=check'. You might
also want to check that the HOMO-LUMO is not causing problems - use
'vshift=x' to increase it. DFT typically gets this separation wrong.

Mark Russell
-----------------------

Thank you guys, I appreciate it very much.

William