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.
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[ output_1.zip ] output files for acetate anion