[Pw_forum] single and triplet states

[ali kazempour]

Dear all
The electronic structure of a   Ga vacancy in GaAs bulk material can be qualitatively understood  in terms of four dangling bonds which are centered at the four As atoms around the missing Ga. In an undistorted situation (Td symmetry) these four dangling bonds give rise to two energy levels : one singlet (a1) and one triplet(t2) . When Ga situated at surface Ga -terminated , the As neighbors becomes three instead of four and four Ga vacancy at surface by  comparison with Ga vacancy at bulk, t2 splitts.
My question is when I simulate the Ga vacancy in the bulk , I only see one defect -induced level  . How can I understand that this level is singlet or triplet?
Also, how can I find the relation between number of dangling bonds around the defects and splitting of the states?
Thanks  a lot 
Ali Kazempour
Physics department, Isfahan University of Technology
84156 Isfahan, Iran.            Tel-1:  +98 311 391 3733
Fax: +98 311 391 2376      Tel-2:  +98 311 391 2375