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Alex Granovsky
gran@classic.chem.msu.su

under some, very rare and specific conditions, Firefly may not
be able to correctly recognize imaginary modes and properly
mark them by the "I" symbol. This is a known bug in the present code.

What one should know is a simple rule stating that Firefly does
not reorder normal modes as obtained after hessian diagonalization,
and that the initially obtained squares of frequencies were arranged
in the ascending order. Firefly takes absolute value of frequencies
and takes a square root of them. Then it deduces imaginary modes and  
marks them accordingly. It also deduces T+R modes during this process.
But it never swap modes and frequencies.

This means that if there is a frequency in the printout followed by a
smaller frequency then the preceding frequency is an imaginary one.

Hope this helps.

Kind regards,
Alex Granovsky


On Thu Sep 25 '14 10:08pm, Owen Williams wrote
----------------------------------------------
>Hi all,

>I'm getting some strange results from a Hessian calculation. The first recorded mode has a frequency  of 537 cm-1, but is not an imaginary frequency. The next five frequencies are very close to zero (  3cm-1), but go up beyond that (i.e. only five rotations and translations were printed). Both the input file header and relevant frequencies are pasted below, and any explanation for why this is the case would be incredibly helpful.

>! HNNS-CO
>!
> $CONTRL SCFTYP=UHF RUNTYP=HESSIAN D5=.TRUE. $END
> $CONTRL MAXIT=480 ICHARG=+1 MULT=1 DFTTYP=PW91 $END
> $FORCE NVIB=2 $END
> $SYSTEM TIMLIM=4800 MEMORY=100000000 AOINTS=DIST $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=454 $END
>
>
>                          1           2           3           4           5
>       FREQUENCY:       537.12        2.97        1.33        0.34        0.19  
>    REDUCED MASS:      4.23899     6.91563    10.88128    11.56489    11.58193
>    IR INTENSITY:      2.70455     0.02222     0.04085     0.06712     0.04527

>       FREQUENCY:         0.26       28.35       48.51       52.46       68.27  
>    REDUCED MASS:      9.01547     5.84289     5.38198     5.72932     8.78352
>    IR INTENSITY:      0.06144     0.02444     0.01232     0.07299     0.01732

>Thank you in advance,
>-Owen Williams