[molpro-user] uccsd(t) energies

Jose A. Gamez joseantonio.gamez at uam.es
Tue Feb 16 11:58:09 GMT 2010

Dear all,

I calculated the energy of the SeCH3 radical at the CCSD(T)/avtz (and 
MP2/avtz) levels and I obtained the following:

              UCCSD(T)          RHF-SCF              UMP2            UHF-SCF
  -2439.71508645  -2439.49831203  -2439.81634192  -2439.50331284

That the UMP2 energy was lower thant the UCCSD(T) one seemed suspicious, and I 
was afraid that the ROHF used as reference of the UCCSD(T) was not the lowest 
state because of the use of the UHF orbitals as initial guess. So I decided to 
repeat the calculation, just the ROHF and UCCSD(T) parts and I obtained the 
same ROHF wavefunction in both cases:

 !RHF STATE 1.1 ENERGY              -2439.498312034134

 !RHF STATE 1.1 ENERGY              -2439.498312034410

and therefore the UCCSD(T) had the same reference, although they lead to 
totally different energies. 

 !RHF-UCCSD(T) STATE 1.1 ENERGY     -2439.860974666455

 !RHF-UCCSD(T) STATE 1.1 ENERGY     -2439.715086449912

The reference energy  and the configuration space for both CCSD(T) calculations 
were the same, so I don't know the origin of this difference. Does any of you 
any explanation for this? I attach both output files for more details.

Thanks for your help



Jose A. Gamez

Departamento de Química, C-13
Universidad Autonoma de Madrid
Cantoblanco, 28049-Madrid. Spain
Phone: +34-91-497-3854
FAX: +34-91-497-5238
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