[molpro-user] strange CCSD(T) results
hinde at ion.chem.utk.edu
hinde at ion.chem.utk.edu
Thu Jun 28 12:55:14 BST 2007
Dear MOLPRO community:
I am a novice MOLPRO user trying to compute three-body terms in
weakly bound van der Waals trimers at the CCSD(T) level of theory.
I have come across some strange results that suggest that I'm doing
something wrong or that there is a bug in the MOLPRO version that
is installed on the machine I'm using (2002.6).
Short form of the problem:
I'm computing total CCSD(T) energies for the systems
A-B-(A) and (A)-B-A
where (A) indicates molecule A replaced by ghost orbitals in order
to perform a counterpoise correction. The underlying symmetry of
the full A-B-A trimer is such that the two systems listed above have
identical geometries. I get the same RHF and MP2 energies for both
systems but different CCSD(T) energies.
Long form of the problem:
Here is the input file in question.
---------- start -----
***,I
memory,45,M;
gprint,orbitals,civector,basis;
geometry={
Z
H1,0, -2.77550006, 0.00000000, 0.00000000;
H2,0, -4.22450018, 0.00000000, 0.00000000;
H3,0, 0.00000000, 3.72097754, 0.00000000;
Cl,0, 0.00000000, 6.12957764, 0.00000000;
H5,0, 4.22450018, 0.00000000, 0.00000000;
H6,0, 2.77550006, 0.00000000, 0.00000000;
}
basis=avtz
dummy,H1,H2
hf;
ccsd(t);
dummy,H5,H6
hf;
ccsd(t);
---------- end -----
This file gives the following results:
CCSD(T) HF-SCF CCSD(T) HF-SCF
-461.48298818 -461.23925352 -461.51582064 -461.23925352
When I replace the last six lines of the above input file with the
following:
dummy,H5,H6
hf;
ccsd(t);
dummy,H1,H2
hf;
ccsd(t);
which simply switches the order of the calculations, I get these
results instead:
CCSD(T) HF-SCF CCSD(T) HF-SCF
-461.51582064 -461.23925352 -461.51582064 -461.23925352
To add to the puzzle, if I replace the basis=avtz line with an
explicit specification of the basis sets (downloaded from the EMSL
library), the first file gives these results:
CCSD(T) HF-SCF CCSD(T) HF-SCF
-461.51582064 -461.23925352 -461.51413322 -461.23925352
and the second file gives these results:
CCSD(T) HF-SCF CCSD(T) HF-SCF
-461.51582064 -461.23925352 -461.51106931 -461.23925352
So you can see that I've gotten four different CCSD(T) energies for
the same molecule, depending on how I do the calculation. Every
calculation gives the same HF (and MP2) energy.
Finally, if I set basis=avdz instead, the two files each give the
same CCSD(T) energy for both species: -461.43754496 hartree.
Any ideas what I might be doing wrong?
Thanks,
Robert Hinde
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