Electron configmation. ground state

A theoretical estimate of the spin spin parameter in the effective Hamiltonian can be made by the use of equations (7.111) and (7.121) using either ab initio or simpler, less accurate wave functions. This was done for the case of O2 in its ground state in an influential paper by Kayama and Baird [39]. They demonstrated that, in the case of a configuration (which occurs quite commonly in practice), there is significant spin-orbit mixing between the lowest Xl state and the metastable S+ state which arises from the same electron configmation. This has the effect of lowering the = 0 component of the state and so makes a positive contribution to the spin... 

Subsequent ab initio calculations by Cocchini et al, however, laid the ground for a different scenario which consists of accidentally small hnear JT couplings and a significant PJT interaction with a nearby nondegenerate (at the Dsft configmation) electronic state. Remember that according to Sec. 2.4 there is no geometric phase effect in such a situation. 

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