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Re^3: Too many iterations in CPHF (MP2 gradient calc)

David Shobe

I have to take some of this back.  I opened the output file in Facio, and all the bonds have been stretched about 2x their normal size.  I don't know how that happened.  Anyway, that likely caused the convergence problem.  I'm deservedly embarrassed about this, but it's hard to know where to look when something goes wrong.

Thanks for everyone's patience,
--David Shobe

On Fri Mar 25 '16 0:49am, David Shobe wrote
>Thanks, Alex.  (although now I look for the "search" button and can't see it).  Anyway, you wrote, at the link provided:

>> These problematic cases should be further inspected to find out
>> whether MP2 problems are caused by the use of inadequate basis
>> set (and hence, the problems can be solved using better basis
>> set),
>> or are inherent to MP2 (e.g., because of milti-configuration
>> nature of true wavefunction).

>I doubt (although I could be wrong) that simple alkynes require a multireference model.  That leaves the basis set.  I think of larger basis sets as having more convergence difficulties but there could be exceptions.  I would have liked a triple-zeta basis set, but I also would like to be able to put in third-row elements (Ge-Br) and options are limited.  

>Anyway, I am only trying to calculate heats of hydrogenation, compared to that of acetylene.  (R-CC-R' + CH3-CH3 --> R-CH2-CH2-R' + HCCH).  Using MP4/DZV(d,p)++//MP2/DZV(d,p) might actually be overkill for an isodesmic reaction like this.  I was in the habit of using CCSD//B3LYP methods (and a different computational chemistry program) for just about everything containing only s- and p- block elements, and it usually worked in a reasonable amount of time.  Now I can't use coupled cluster theory with your program (correct?) so I have to be a bit more judicious.

>--David Shobe
>On Thu Mar 24 '16 10:11pm, Alex Granovsky wrote
>>Dear David,

>>if you search this site (using "search" button on the left) for mp2 cphf
>>you will find the following link


>>Hope this help.

>>Kind regards,
>>Alex Granovsky

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