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Alex Granovsky
gran@classic.chem.msu.su

regardless on whether this particular imaginary frequency
is artifact or not, the better way to go away is to compute
normal modes for all atomic masses set to unity, then go away
along the computed imaginary mode as you described. You may need
to use larger multipliers though. The reason why one needs to set
masses to unity is that this will search for imaginary mode of
PES itself, rather than for imaginary mode of atomic vibrations.

Kind regards,
Alex Granovsky



On Mon Dec 24 '12 5:55am, Panwang Zhou wrote
--------------------------------------------
>Dear all,

>Recently I carried out a geometry optimization using MCSCF(16,14) and then performed frequency calculation on the optimized geometry. The result indicated that there is one imaginary frequency (39.56I), then I generate the new coordinates with the following method:

>"Optimized Coordinates" + 0.5*"Imaginary Frequency" (Usually this is useful in Gaussian)

>and then re-optimized the geometry and perform frequency calculation, however, the imaginary frequency still existed (39.66I). I repeat the process again and I still get an imaginary frequency (40.27 I).

>It seems that the aboved method could not remove the imaginary frequency in Firefly. So, what's the correct way to remove imaginary frequency in Firfly? How to ajust the optimized geometry? Thanks.

>My input file：
>!   File created by MacMolPlt 7.4.3
> $CONTRL SCFTYP=MCSCF RUNTYP=OPTIMIZE EXETYP=RUN MAXIT=50 ICHARG=-1
>    MULT=1 FSTINT=.T. GENCON=.T. D5=.T. INTTYP=HONDO NOSYM=1
>    ICUT=11 ITOL=30 $END
> $SYSTEM MWORDS=300 TIMLIM=60000.0 KDIAG=0 NOJAC=100 $END
> $SCF DIRSCF=.T. FDIFF=.F. NCONV=8 $END
> $P2P P2P=.T. DLB=.T. $END
> $TRANS MPTRAN=2 DIRTRF=.T. AOINTS=DIST ALTPAR=.T. MODE=112 $END
> $MCSCF CISTEP=ALDET FULLNR=.F. SOSCF=.T. $END
> $DET NCORE=49 NACT=14 NELS=16 $END
> $BASIS EXTFIL=.T. GBASIS=TZVP $END
> $GUESS GUESS=MOREAD NORB=642 $END
> $STATPT NSTEP=200 METHOD=GDIIS OPTTOL=0.00001 HSSEND=.T. $END
> $FORCE NVIB=2  $END
> $DATA
>Title
>C1
>C    6.0        2.7132783145    -1.4740302589   -.0597953357
>C    6.0        4.0072333395    -1.0587405339   -.0523177056
>C    6.0        4.3509360614    .3549750338     .0047198534
>C    6.0        3.2110719721    1.2609116970    .0544028295
>C    6.0        1.9242681910    .8199231769     .0449917837
>C    6.0        1.5962366929    -.5763413550    -.0135105729
>H    1.0        2.5090291816    -2.5321904000   -.1049181082
>H    1.0        4.8194252269    -1.7639108567   -.0902441560
>H    1.0        3.4295424902    2.3141043298    .1003293599
>H    1.0        1.1243498618    1.5313547817    .0836053890
>C    6.0        .3110181302     -1.1305273538   -.0277143026
>H    1.0        .3099287926     -2.2086654130   -.0629691982
>C    6.0        -.9868693392    -.6482779462    -.0084178737
>C    6.0        -1.5542949968   .6857027999     .0186426827
>C    6.0        -3.1400841402   -.9091688721    -.0109188077
>N    7.0        -2.0507028080   -1.5755767961   -.0262051103
>N    7.0        -2.9262976042   .4589864153     .0171317950
>O    8.0        -1.0737305753   1.7997169650    .0373172595
>C    6.0        -3.9107928678   1.4991176546    .0280233794
>H    1.0        -3.3895141243   2.4438580486    .0569467246
>H    1.0        -4.5338570060   1.4747095620    -.8621195135
>H    1.0        -4.5568760150   1.4328337955    .8991551602
>C    6.0        -4.5112041528   -1.4987819631   -.0176023140
>H    1.0        -4.4221489875   -2.5756182595   -.0409245327
>H    1.0        -5.0740697743   -1.2147434323   .8678104944
>H    1.0        -5.0821323510   -1.1773397307   -.8848586971
>O    8.0        5.5107321775    .7514431412     .0110739666
> $END
>--- OPTIMIZED MCSCF MO-S --- GENERATED AT  0:43:41 19-DEC-2012
>Title
>E(MCSCF)=     -719.8906562926,  0 ITERS, E(NUC)=  996.3878966217
> $VEC
> 1  1-1.36077782E-06 6.69461457E-06-1.32096704E-04 7.44449420E-04 9.20147697E-04
>....
> $END