Ceilidh ... Re: MP2 geo opt convergence with imaginary frequency problem (using ICUT=25-30, ITOL=30, INTTYP=HONDO, NCONV=10)

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Re: MP2 geo opt convergence with imaginary frequency problem (using ICUT=25-30, ITOL=30, INTTYP=HONDO, NCONV=10)

Hi!
Sometimes it is necessary to overthrow the starting geometry in other valley - what leads to a local minimum instead of to a saddle point. An example of calculation for your structure is applied. However in this run symmetry of structure has strongly decreased.
Best regards,
Vyacheslav
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On Mon May 17 '10 8:10am, Gordon wrote
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>Dear forum followers,

>I have been optimising geometries to calculate gas phase proton affinities with FF, using MP2 6-311+G(d,p) which will generally yield calc'd values within +/- 10 kJ/mol of experimental values. Now I am trying the same business using $BASIS NDFUNC=2 NFFUNC=1 NPFUNC=2 DIFFSP=.TRUE. DIFFS=.TRUE. $END. With this I can optimise the geometry, BUT, I also observe a single imaginary frequency. To try to fix this, I then used the same input and included $CONTRL RUNTYP=Optimize ICUT=30 ITOL=30 INTTYP=HONDO $END and $SCF DIRSCF=.TRUE. NCONV=10 FDIFF=.FALSE. $END. (I have attached the two outputs for the acetate anion here). I have also tried with ICUT=25, but I obtain the same result.

>I am not sure what I could do now to get further down the PE well! Perhaps directed adjustment of these ICUT, ITOL, NCONV values are in order? After 3 tries I was hoping to receive a suggestion from those who are more knowledgeable. Perhaps there is another trick to try?

>I would be most grateful for any suggestions.

>Cheers,
>Gordon

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