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Alex Granovsky
gran@classic.chem.msu.su

I'm sorry for long delay with my reply.

>All imaginary frequencies are < -35 cm-1. Is this a problem with the accuracy of the Hessian?

For imaginary frequencies in this range the suggestions made by Pavel
and Siddheshwar can help. Note, as frequencies are computed semi-
numerically, there is some numerical noise in their values which is
most prominent in the six "zero" frequencies. This noise can result
in small negative (i.e. imaginary) frequencies to be computed instead
of correct zero (or very small positive) values.

Kind regards,
Alex Granovsky

On Wed Jun 11 '14 3:34pm, Owen Williams wrote
---------------------------------------------
>Prof. Granovsky,

>All imaginary frequencies are < -35 cm-1. Is this a problem with the accuracy of the Hessian?

>Thank you everyone for all the suggestions,
>-Owen

>On Tue Jun 10 '14 8:45pm, Alex Granovsky wrote
>----------------------------------------------
>>Dear Owen,

>>how large are the imaginary frequencies?

>>Kind regards,
>>Alex Granovsky
>>
>>
>>On Tue Jun 10 '14 7:17pm, Owen Williams wrote
>>---------------------------------------------
>>>Dear Firefly Users,

>>>I am working on a rather large organometallic molecule (94 atoms, two Rh centers) and am having trouble removing imaginary frequencies from the Hessian calculation. The original optimization input is shown below and results in four imaginary frequencies. Tightening the convergence criteria to 0.00001 resulted in a Hessian with three imaginary frequencies. Do I need to continue to tighten the convergence criteria and if so, what value for OPTTOL would ensure no imaginary frequencies? I only ask because the Hessian is so expensive (~7 days) and I would rather only calculate it once (more).

>>> $CONTRL SCFTYP=UHF RUNTYP=HESSIAN D5=.TRUE. ECP=read $END
>>> $CONTRL MAXIT=480 ICHARG=+2 MULT=1 DFTTYP=PW91 $END
>>> $CONTRL INTTYP=HONDO ICUT=11 ITOL=30 $END
>>> $SYSTEM TIMLIM=10000 MEMORY=100000000 AOINTS=DIST $END
>>> $STATPT HSSEND=.T. NSTEP=200 OPTTOL=0.0001 $END
>>> $SCF DIRSCF=.T. FDIFF=.F. NCONV=8 DIIS=.T. $END
>>> $P2P P2P=.T. DLB=.T. MXBUF=2048 $END
>>> $INTGRL PACKAO=.T. $END
>>> $GUGEM PACK2=.T. LCHAIN=0 $END
>>> $SMP call64=.T. $END
>>> $GUESS GUESS=MOREAD NORB=1054 $END

>>>Thank you for all your help,
>>>-Owen Williams