Firefly and PC GAMESS-related discussion club



Learn how to ask questions correctly

Re^6: catalysis, negative energy activation in MP2 calculation

sanya
sanya@photonics.ru

Thank you. I see, the sum of the reagents + catalyst energies is really higher than the energy of the TS by ~9 kcal/mol. Try the following:

(1) increase the accuracy of the calculation by setting INTTYP=HONDO (in $CONTRL group), OPTTOL=0.00001 (in $STATPT group). Set HESS=Guess in $STATPT group for the optimizations of the reagents and catalyst and, since the hessian is calculated numerically in MP2, set NVIB=2 in $FORCE group for all calculations. Probably, your geometry optimizations will converge to better values. In addition, you'll obtain better frequencies.
(2) One of the sources of errors is BSSE (Basis Set Superposition Error). Increase the basis set (to at least triple-zeta quality, such as 6-311G** or, better, cc-pVTZ). Use BSSE correction schemes (unfortunately, I cannot give you any working example of a BSSE calculation).
(3) It is known that MP2 gives wrong stationary points in some cases (say, TS instead of minima or vice versa). Increase the PT level (MPLEVL=4) and perform single-point runs for all the structures (reagents, catalyst, and TS). Probably, your TS turns out to be a minimum at the MP4 level of theory.

As an alternative to MPx, you may try DFT and compare the results.

On Wed Feb 24 '10 6:15pm, Ronald Conol wrote
--------------------------------------------
>I've already attached my reactants output file as well as the catalyst.. I hope you can take a look at it and find out that the energy or the transition state is much lower than the sum of the reactants and catalyst that is bothering me right now.

[ Previous ] [ Next ] [ Index ]           Thu Feb 25 '10 2:34am
[ Reply ] [ Edit ] [ Delete ]           This message read 799 times