[molpro-user] Strange CCSD(T) behavior

Steven E. Wheeler swheele2 at chem.ucla.edu
Wed Dec 24 17:43:19 GMT 2008

This seems to have been a compilation/hardware issue, as I am now 
getting correct (T) corrections on a different cluster with the same 
input file.

Thanks for everyone's suggestions though.


Gerald Knizia wrote:
> On Friday 19 December 2008 17:39, Steven E. Wheeler wrote:
>> I am running into some strange (T) behavior for an open-shell (doublet)
>> system.  For certain geometries (for example, displaced geometries in a
>> finite difference frequency or optimization calculation) I get a (T)
>> correction that is about three times the size of the (T) corrections at
>> nearby geometries. [...]
>> I'm not convinced this is a Molpro issue (running tests now with ACES),
>> but wanted to throw it out on the list to see if anyone had any ideas.
>> I have checked for orbital instability issues but have found none.
> I personally have never seen such behavior. But if you would provide the full 
> input files, I could have a quick look.
> Some wild speculations on that:
>   o I once saw problems when trying to disable integral compression. Did you 
> try the calculation with something like 
>   gthresh,energy=1.0d-12,orbital=1.0d-12,oneint=1.0d-16,twoint=1.0d-14
> (note the non-present compress threshold) instead of the
>  gthresh,energy=1.0d-12,orbital=1.0d-12,oneint=1.0d-16,twoint=1.0d-16,
>     compress=1.0d-16
> you gave? Of course this should work, but since tweaking the compression 
> accuracy it is a rarely used feature, there is a good chance that occassional 
> problems might go by undetected.
> About the orbital threshold: We usually use {rhf; accu,18} or similar when 
> trying to converge RHF to the limit. But both should work.
>   o Sometimes compiler errors occur. In particular, I've seen various versions 
> of ifort 10.x generate very wrong code in several occasions. Of course this 
> is just a wild guess, but you could try recompiling MOLPRO with a different 
> compiler, or compiling tripu.f and uccsd.f with lower optimization settings. 
> Did you run "make test"?
>   o If running in multiprocessor mode: does it also occur in serial mode?
>   o Just to make sure: You did check that the calculated electronic states are 
> equal in all cases, or? (I.e., no different states in distorted geometries 
> due to changes in molecule symmetry, RHF dipole moments are similar and so 
> on).

Steven E. Wheeler
Postdoc, Houk Group
Department of Chemistry
University of California, Los Angeles

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