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Re^2: imaginary frequency along H-bond, how to get rid of it?

olgerdovich
olgerdovich@yandex.ru


Thank you for your kind reply.
To my deep regret, your advice doesn't work.

To check your solution, I run nitromethane radical-anion hydrate task which is somewhat smaller and less resource-demanding. It has an imaginary frequence of -32.5 when optimized in 6-31++G** set at U-B3LYP level.

Putting in new input its coordinates and $VEC and $HESS groups, running runtyp=irc with $irc forwrd=.t. npoint=30 $end , I've got


CURRENT ENERGY=     -321.5108901568, IS HIGHER THAN THE
   LAST ENERGY=     -321.5108901801
YOU ARE NO LONGER ON A STEEPEST DESCENT PATH!
YOU MAY JUST HAVE PASSED THROUGH A MINIMUM

ADDRESS 0x007292E4 HAS INITIATED PROGRAM ABORT BECAUSE OF FATAL ERROR(S)

.....

EXECUTION OF FIREFLY TERMINATED ABNORMALLY


The same holds for  $irc forwrd=.f. npoint=30 $end (result is essentially identical in step number, final energies and geometries)
Further,  $irc PACE=RK4 stride=0.01 forwrd=.t. npoint=30 $end doesn't work too:


CURRENT ENERGY=     -321.5108901522, IS HIGHER THAN THE
   LAST ENERGY=     -321.5108901801
YOU ARE NO LONGER ON A STEEPEST DESCENT PATH!
THIS IRC SHOULD BE RUN WITH SMALLER STEPS AND/OR A BETTER INTEGRATION METHOD.

ADDRESS 0x0072C3FA HAS INITIATED PROGRAM ABORT BECAUSE OF FATAL ERROR(S)

......

EXECUTION OF FIREFLY TERMINATED ABNORMALLY


So, what should I do next?

(btw: how do you extract quite large $VEC group from PUNCH-file? It is inconvenient to copy it from text editor, and ChemCraft returns only occupied orbitals rounded to 1e-6. It caused no differences when I tried either variant and both led to the same result, but I'm not sure which way is more righteous?)








On Tue Jul 7 '15 5:29pm, Pedro Silva wrote
------------------------------------------
>On Mon Jul 6 '15 3:39am, olgerdovich wrote
>------------------------------------------
>>Dear collegues,

>>During geometry optimization of a complex of trifluoronitromethane anion-radical and water molecule, I've got a TS-like structure of approx. Cs-symmetry (as it looks). I can't get rid of this im. freq. and ask your kind advice.

>>Even use of very tight convergence criteria (OPTTOL=0.000001, nconv=8) still left it. Similarly, small random displacement of oxygen and hydrogen atoms does not solve the problem, as well as displacement along imaginary oscillation (by catching most diturbed structure while visualizing this oscillation in Chemcraft, neither with 1-x displacement scale nor with 0.05-x displacement scale). Calculating of starting hessian also doesn't help.
>
>
>You can move away from that TS by runnning an IRC computation:
>1) Paste the final coordinates, $VEC group and $HESS in your input file
>2) set runtyp=irc
>3) include $irc forwrd=.t. npoint=30 $end

>If this run does not afford your intended gemoetry, you may follow the imaginary frequency in the opposite direction by replacing step 3 by

>3A) include $irc forwrd=.F. npoint=30 $end
>
>
>Pedro S.


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