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

Pedro Silva

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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