PC GAMESS/Firefly-related discussion club

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Alex Granovsky
gran@classic.chem.msu.su

Actually, Sergey provided me the link to his output file.
It is very large (ca. 500 MB) and has some non-standard settings:

$STATPT NPRT=1 NPUN=2 $END

After grepping it for the string "E(MP2)" and manually
plotting energy graph I got the picture below. Evidently,
ChemCraft does not correctly recognize the target energy...
and plots RHF energy change instead!

To Sergey: you can file bug report to ChemCraft author...

regards,
Alex




On Sat Nov 14 '09 8:26pm, Sergey wrote
--------------------------------------
>Dear all,

>I try to optimize adenine-adenine stack minimum using MP2/cc-pVTZ (pls. see att.1, att. 2), and I witness unexpected behaviour. As this system is a weakly, dispersion bound di -molecular system, the overall energy is 2*(adenine)+dispersion energy, and in the beginning of optimization, the overall energy quickly drops - as mainly molecular but not intermolecular geometry is optimized.

>Thus, pretty soon we have a minima which corresponds to 2 separate optimized molecules of adenine that 'do not feel' each other so far. As optimization proceeds to lower gradients (frequencies of Hessian), stacking becomes evident and 2 adenine molecules attract. MP2 overestimates stacking energy so instead of 3.4 A it goes to 3.2 A.

>However, we have an issue: as 2 adenines approach each other, they perturb each other MO geometry, so the energies of single adenines rise; while stacking energy lowers; at some point they should find balance? But if you take a look at the optimization energy vs. cycle graph (attached), you find it climbs up and only change this tendency over large number of cycles when (see attached animation) a mutual orientation of adenine molecules also changes.

>The question is, why the algorithm climbs up for such the extended number of steps? I supposed, it is BFGS -driven code, i.e. it finds minima of energy on PES eventhough the gradient may rise. Indeed, how the algorithm resolve such the situation, where there are 2 coupled, but of different order, contributors to the overall energy term?

>Thank you!  

MP2 Energy graph