Subject: SUMMARY: G98: opt=stable and freq

From: DE BRUIN theodorus <[email-masked]>
Date: Tue, 27 Aug 2002 14:00:01 +0200

Dear All,

I would like to thank everybody who replied to my question concerning the
instability of the wavefunction and the frequency analysis that I wanted to
perform subsequently.  
I wish to point out that the instability did not have a RHF/UHF origin,
since I used (of course) uB3LYP. 
Initially I used a --link1-- job with guess=read geom=check freq.  However
in that case I fell back on the old(=instable wavefunction). In my case the
combination: guess=(read,mix) geom=check freq 
resulted in a freq analysis on the stable wavefunction.

Thanks again.
Theo

REPLIES:

Frank Jensen [[email-masked]]:
Try generating the stable wave function (= the one from
stable=opt) by e.g. guess=alter or guess=mix. This works if the
wave function is RHF --> UHF unstable. If the instability is
a symmetry breaking you may need to create your own route card
where the wave function is first done with stable=opt, and then
immediately continuing onto the frequency. If that is necessary,
however, it strongly suggests some (other) problem with your
wave function, perhaps it is strongly MCSCF, or the DFT grid is
insufficient.

Monica Kosa [[email-masked]]:
You can try read optimized wavefunction from the checkpoint file.
First you run a job with stable=opt keyword. Than you include the keyword
freq (and delete the stable=opt) and guess=read (in the same input file you
ran your stable =opt job). 

Reinaldo Pis Diez [[email-masked]]:
I posted a very similar question even before, in 1999!!
Below I send you the "official" answer, that is, that of Gaussian (my 
original mail is included, too). Note that the suggested recipe is the same 
you tried!! 	
Note also that an alternative three-step job is proposed: use first 
stable(opt); then use guess(read) opt to get the true minimum; and finally 
do the freq calculation.
Dr. Diez,
There are a couple of points. First, you want to use GUESS=READ and the
checkpoint file from the STABLE=OPT calculation to have a chance of
recovering
the final stable solution. In some nasty cases you may need to use SCF=QC as
well. Second, note that if the SCF was unstable for the optimization then
the
structure no longer corresponds to a minimum and you need to re-optimize to 
get
the true minimum energy structure. You might want to consider changing your
protocol and do a STABLE=OPT at the first point, then use GUESS=READ OPT
with
SCF=QC if needed. This should get you the stable SCF solution to start with
and the go from there.
 > Dear CCL'ers,
 >
 > I'm doing some calculations (geometry optimizations) on small systems
 > using DFT as implemented in Gaussian 94 and 98.
 > Once the geometry is optimized, I do a stable(opt) job on the wave
 > function corresponding to that geometry. I find very often that the wf is
 > unstable, but the opt option in the stable keyword works well and
overcome
 > the unstability problem.
 > Then, I try a frequency calculation using guess(read) and geom(check) but
 > the calculation is performed on the unstable wf instead of the stable
one!!
 > How can I perform the frequency job using the stable wf? Which
 > combination of keywords am I missing?
 > Thanks in advance.
 > Regards,
 >
 > Reinaldo

Laurence Cuffe [[email-masked]]:
I'm only stepping in with advice here as you say a similar question 
in the archives remains un-answered.  I would use the final 
geometry from the stable calculation to start an opt=(calcall) 
calculation with the multiplicity in the input file corrected to 
correspond to that derived from the stable calculation.  
Hope this helps

Christopher Cramer [[email-masked]]:
   We work with unstable wave functions all the time. We find it much more
efficient to simply run a "stable" calculation (i.e., no "=opt"). If there
is indeed an instability (which the output will tell you), then we proceed
to

# u[dft method]/[basis] guess=(read,mix) scf=([qc,nosym,direct,cycles=900])
 [fopt] [freq]

where the brackets indicate wildcards (e.g., the particular DFT method and
basis set) or options that may or may not be required to coax the scf to
convergence (qc is helpful for difficult cases, nosym is essentially
required for molecules that are not C1, but is better used within scf, so
that geometry optimization can still take advantage of symmetry, direct may
not work depending on the size of the system, number of cycles is a
personal choice...) In this case, you can also add optimization and
frequency keywords. Note that it is CRITICAL to include the "u" prefix to
the DFT method.

   I seem to recall that the trouble with your approach is that stable=opt
usually begins with a restricted wave function, and then if an instability
is found, an unrestricted wave function is optimized (although often it
seems unable to accomplish this, while guess=mix as listed above is quite
robust). HOWEVER, when link 1 is entered again because of your use of the
keyword freq, it defaults back to a restricted formalism, so you simply
guess the alpha orbitals as an RDFT wave function. My memory may certainly
be wrong on this particular point -- I haven't tried it in a long time --
but we usually have excellent success with the protocol I've outlined
above.

[email-masked]
     A two-step calculation is necessary. In the second job, the 
frequency part, use "ub3lyp geom=allcheckpoint guess=read nosymm". If 
you do not explicitly specify the "u" in ub3lyp, the calculation will 
converge to the spin-restricted wavefunction. If this doesn't help, 
please send me your input file and I will attempt to find a set of 
options for you.

Regards,
Jim Hess
++++++++++++++++++++++++++
James Hess, Ph.D.
Customer Support Scientist
Gaussian, Inc.
e-mail: [email-masked]
++++++++++++++++++++++++++

Stock [[email-masked]] :
You should be able to get the optimized wavefunction with appropriate 
use of Guess.  Try Guess=Mix, which mixes the HOMO and LUMO.  If that 
doesn't work, check out Guess=Alter, G98 user's reference pp. 98 and 
101.  This will allow you to specify by hand which MOs you want 
occupied.  I'm unfamiliar with the notation used in the manual, so if 
you end up taking this route and you are familiar with it, I'd be much 
obliged for an explanation.  Perhaps a #P Stable calculation would give 
you the info you need to correctly choose the alterations to the 
wavefunction.
While you're trying to create the stable wavefunction, you can of course 
test it with another stable calculation.

Juan Pablo Senosiain [[email-masked]]:
it sounds like you might have a UHF/RHF instability.  This is well
documented in standard quantum chemistry textbooks.  Try the homolytic
bond fission using UB3LYP.

Patrick Musch [[email-masked]]:
as far as I know, the SCF routine in Gaussian uses quatrically
convergent (QC) SCF, so try to read in the stable wavefunction from the
checkpoint file and do a Link1 for the frequency job GUESS=READ and
SCF=QC. This might take a little time, since QC SCF is more
time-consuming than the ordinary algortihm. I hope this might work.

Olga Dmitrenko [[email-masked]]:
Did you try to do first job with opt=stable and link it with the
control card # guess=read geom=check freq ?
We are doing similar studies on O-O bond elongation in peroxides.
Our way is to do UB3LYP with guess=mix.

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