| 4:55:18 PM PDT - Wed, Apr 12th 2017 |
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Hi Marcel,
This means that the (A-B) is not positive definite and that suggests that the singlet ground state
in your calculation is unstable to a triplet. The consequence of this is there are -ve excitations.
A workaround is via the Tamm-Dancoff approximation (keyword tda). This may solve the problem.
But I think the issue in your case is the singlet-triplet instabiity as I suggested earlier.
Feel free to send me a direct email if you want to discuss more.
Best,
-Niri
niri.govind@pnnl.gov
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