Query related Open-shell singlets calculation in ATK

[Jyotirmoy Deb]

I have optimized an 2D structure in which three different cordination of carbon is present.  In case of three cordinated carbon we are expecting two single bond and one double bond but unfortunately we are getting some different situation.  One reason may be the wave function is an open-shell singlet. Whether open-shell singlets can be performed using ATK for periodic structures and how can I get rid of this problem?

[Petr Khomyakov]

QuantumATK methodology for electronic structure calculations is based on density functional theory (DFT), so there is no many-body wave function, as the method is based on an alternative description of many-body problem, density functional. So, in that sense, one can not do, e.g., restricted Hartree–Fock (RHF) method for open-shell singlets, as it is not implemented  in QuantumATK.

But three-coordinated carbon atoms as such are pretty common to be handled with DFT, e.g., in graphene or carbon nanotube related systems, or benzine molecule or similar. Could you post your python script and related log file for this calculation to see your actual system and settings? 

[Jyotirmoy Deb]

Thank you for kind reply. I totally agree with you that DFT can  handled three cordinated C atom. But in my case the problem is that the bond lengths between C2 atom with its three neighbours are respectively 1.36, 1.42 and 1.42 Angstrom (please see the attched figure)  respectively, which indicates one double bond and two roughly 1.5 times bonds and as a reasult total coordination changes to more than 4 instead of 4. I have performed the calculations using both PBE and HSE functional and the results are more or less same. Can you please suggest what may be the possible reason and how can I overcome this situation. Any kind of help is highly appreciated.
Actually I have communicated this structure in a prestigious journal and reviewer raised question here  for that reason I am not able not to give the log file at this moment.

[Jyotirmoy Deb]

I am attaching here one more clear view of the structure for your reference.

[Petr Khomyakov]

How do you evaluate that the bond is 1.5? To my understanding, you have got 2 bonds shorter than the 3rd one, kind of consistent with 1 double + 2 single bond picture.

[Jyotirmoy Deb]

I have not evaluated that the bond is 1.5. Actually I want to mean that the bond length for a single bond is usually 1.54 Angstrom and that of a double bond is 1.34. In our case the bond lengths between C2 atom with its three neighbours are respectively 1.36, 1.42 and 1.42 Angstrom (please see the attched figure)  respectively. i.e. one double bond and two bonds have higher than single bond type and lower than double bond. Thus total no of bonds exceeds 4 which is not possible. Expected bond lengths between C2 atom with its three neighbours are 1.34, 1.54 and 1.54 Angstrom but in our case situation is different.