[molpro-user] MOLPRO and CCSD(T)

[Kirk Peterson]

Arvi,

The difference is that G03 uses UHF-UCCSD(T) for open shell species,  
whereas Molpro uses ROHF-UCCSD(T) [or ROHF-RCCSD(T) depending on if  
you chose uccsd(t) or rccsd(t), respectively[.  Even though you've  
evidently used UHF orbitals in Molpro, it presumably only took the  
alpha set and employed these as if they came from a ROHF  
calculation.  This will lead to the 2 mEh energy difference. There  
are no post-UHF methods implemented in Molpro.  BTW, in Molpro  
"closed-shell" orbitals are still correlated.

-Kirk

--------------------------------------------
Kirk A. Peterson
Professor of Chemistry and Materials Science
Washington State University
Pullman, WA 99164-4630

Office: (509) 335-7867
Fax:    (509) 335-8867
kipeters at wsu.edu
http://tyr0.chem.wsu.edu/~kipeters/
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On Jan 13, 2006, at 2:35 PM, Arvi Rauk wrote:

>   We recently installed the latest MOLPRO and I ran my first CCSD 
> (T) calculation (on a test molecule, dimethylsulfide radical  
> cation).  Although the run was about 5 times faster that with G03  
> with the same geometry and basis set, the CCSD(T) energies were  
> different (MOLPRO's is higher by 0.002 au - the UHF energies are  
> the same). Comparing the two outputs, it appears that G03 uses all  
> of the 19 valence unrestricted orbitals as "active" but MOLPRO  
> describes 9 valence orbitals as "closed-shell" and only one orbital  
> as "active".  Is this really the source of the difference?  If so,  
> is there any way to reproduce G03's CCSD(T) results?  I've poured  
> through the manual and can't see any way.
>   Hope it's not too naive a question.  We've been doing a lot of  
> CCSD(T) calculations with G03 and obtained MOLPRO because of its  
> reputation for being significantly faster.  We'd obviously like to  
> merge the two sets of calculations.
>   Arvi Rauk
>