[molpro-user] ScO bond length and basis sets

[Kucukbenli Emine]

Dear All,

I am not sure if this is the appropriate place for this question but I would be very happy if you could find the time to answer:

I am trying to determine the bond length of Scandium oxide dimer at its ground state. The ground state is known to be a sigma state with 1 unpaired electron. The experimental bond length is
1.668 Angstrom.

There is a very nice paper on the whole 3d transition metal oxide dimers by Bauschlicher and Maitre (http://link.springer.com/article/10.1007%2FBF01113847) that reports the bond length with UCCSD(T) as
1.680 Angstrom.

I perform UCCSD(T) calculations as well, and as they did, I also correlate 3s and 3p electrons using the cc-pwcvXz kind of basis sets and using the "core" directive.

That paper also suggest that at the MCPF level of theory, relativistic effects are very small. To account for them anyways, I use basis sets that are like cc-pwcvXz-DK (but I do not add any other directive to the input file - is it necessary?)

Considering it is an ionic environment for oxygen, for it I use the same basis set but augmented with diffuse functions, such as (aug-cc-pwcvXz-DK). In Bauschlicher work the diffuse g function is deleted but I don't know why that is done or how, so I simply used the aug- basis set as is.

My resulting input files look like the following:

***,ScO
geometry={sc;o,sc,r}
r=1.66 ang
basis,sc=cc-pwcvqz-DK,o=aug-cc-pwcvqz-DK
{rhf; wf,29,1,1}
{uccsd(t); occ, 9,3,3,0; closed, 8,3,3,0; core, 4,1,1,0;}
{optg, GRADIENT=1d-5, ENERGY=1d-6}

The geometry optimization yields the following results:

basis set  vs  bond length in angstrom
TZ : 1.657 A   #cc-pwcvTz-DK/aug-cc-pwcvTz-DK
QZ : 1.659 A    #cc-pwcvQz-DK/aug-cc-pwcvQz-DK

Before going and doing the time consuming 5Zeta basis set calculation I wanted to understand whether I am doing something wrong here, as my results are far from the previous calculations of Bauschlicher and Maitre.
So I tried to following to see the convergence behaviour of the basis set:
{uccsd(t); occ, 9,3,3,0; closed, 8,3,3,0; core, 6,2,2,0;}
i.e. using the same basis set but not correlating the 3s and 3p; for which the calculations are cheaper.
This is what I get in this case:

basis set vs Bond length in Ang.
Tz:  1.680   #cc-pwcvTz-DK/aug-cc-pwcvTz-DK
Qz:  1.689   #cc-pwcvQz-DK/aug-cc-pwcvQz-DK
5z:  1.678   #cc-pwcv5z-DK/aug-cc-pwcv5z-DK

Can this even be concluded to be a convergent behaviour?

Being quite inexperienced in both the theory and the code I cannot disentangle whether it is a problem on the theoretical level or my usage of the code.
If you could point to what I am doing wrong here I would be very happy!

Many thanks in advance,

Emine Kucukbenli,
Institute of Materials, EPFL, Switzerland
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