Unrestricted DFT in H2 Dissociation

[Wook]

Hi all,

I am trying to compute a H2 dissociation curve using UBLYP/SVP level of theory.
When the distance is far enough (~ 10 angstrom), closed shell with restricted BLYP and triplet with unrestricted BLYP give reasonable energies.
However, open shell singlet gives the same energy as the restricted one or never converge if electrons are assigned manually into two different orbitals ( ex, a 1, b 2 in define).
I expect that open shell singlet should give the same energy as triplet, and I am wondering whether I made something wrong in setting up the open shell singlet calculation.

I would appreciate any comments.

Thanks,
Wook

[christof.haettig]

I'm not sure what you want to compute. But I have the impression that you tried to converge the SCF procedure to an excited state by asking the program to occupy for beta spin the 2nd orbital and leaving the lowest energy beta spin orbital empty. This is not supposed to work.

If you want to compute the correct (symmetry broken) spin unrestricted ground state for H2 1.) switch off symmetry and 2.)  generate somehow
symmetry broken start MOs. You can do the latter by modifying by hand for one of the occupied orbitals the coefficients in the alpha or beta file
or you can do a prelimary calculation where you change the charge for one of the atoms a little bit.
For a scan of the potential curve start at large distances and then proceed to smaller distances. (Not the other way.)

[Wook]

Dear Christof

Thank you very much for your reply.
I am still wondering whether there is a way to do what you said by a command in Turbomole, like 'guess=mix' in Gaussian.
Is it possible?

Thanks,
Wook

[christof.haettig]

I've not used Gaussian since more than 20 years.... No idea what 'guess=mix' in Gaussian does.
That symmetry has to be broken 'by hand' to get the correct UHF solution is a very special problem for higly symmetric molecules. Up to know I had this only for H2 which I use to teach students the bond breaking problem in HF and KS-DFT.

Christof

[Wook]

Thank you for your reply!

Wook