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<div style="direction: ltr;font-family: Tahoma;color: #000000;font-size: 10pt;">Dear All,<br>
<br>
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:<br>
<br>
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
<br>
1.668 Angstrom. <br>
<br>
There is a very nice paper on the whole 3d transition metal oxide dimers by Bauschlicher and Maitre (<a href="http://link.springer.com/article/10.1007%2FBF01113847" target="_blank">http://link.springer.com/article/10.1007%2FBF01113847</a>) that reports the
bond length with UCCSD(T) as <br>
1.680 Angstrom. <br>
<br>
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.<br>
<br>
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?)<br>
<br>
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.<br>
<br>
My resulting input files look like the following:<br>
<br>
***,ScO<br>
geometry={sc;o,sc,r}<br>
r=1.66 ang<br>
basis,sc=cc-pwcvqz-DK,o=aug-cc-pwcvqz-DK<br>
{rhf; wf,29,1,1}<br>
{uccsd(t); occ, 9,3,3,0; closed, 8,3,3,0; core, 4,1,1,0;}<br>
{optg, GRADIENT=1d-5, ENERGY=1d-6}<br>
<br>
The geometry optimization yields the following results:<br>
<br>
basis set vs bond length in angstrom<br>
TZ : 1.657 A #cc-pwcvTz-DK/aug-cc-pwcvTz-DK <br>
QZ : 1.659 A #cc-pwcvQz-DK/aug-cc-pwcvQz-DK<br>
<br>
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.<br>
So I tried to following to see the convergence behaviour of the basis set:<br>
{uccsd(t); occ, 9,3,3,0; closed, 8,3,3,0; core, 6,2,2,0;}<br>
i.e. using the same basis set but not correlating the 3s and 3p; for which the calculations are cheaper.
<br>
This is what I get in this case:<br>
<br>
basis set vs Bond length in Ang.<br>
Tz: 1.680 #cc-pwcvTz-DK/aug-cc-pwcvTz-DK <br>
Qz: 1.689 #cc-pwcvQz-DK/aug-cc-pwcvQz-DK<br>
5z: 1.678 #cc-pwcv5z-DK/aug-cc-pwcv5z-DK<br>
<br>
Can this even be concluded to be a convergent behaviour?<br>
<br>
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.
<br>
If you could point to what I am doing wrong here I would be very happy!<br>
<br>
Many thanks in advance,<br>
<br>
Emine Kucukbenli,<br>
Institute of Materials, EPFL, Switzerland<br>
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