First-derivative coupling matrix electronic states

As an example, in a four-electronic-state problem (n = 4) consider the electronic states i = 2 and f = 4 along with the first-derivative coupling vector element Wj4 (Rl) that couples those two states. The ADT matrix ui.4(qx) can... 

The ADT matrix for the lowest two electronic states of H3 has recently been obtained [55]. These states display a conical intersection at equilateral triangle geometi ies, but the GP effect can be easily built into the treatment of the reactive scattering equations. Since, for two electronic states, there is only one nonzero first-derivative coupling vector, w5 2 (Rl), we will refer to it in the rest of this... 

The vector w(1,ad(qjJ [or wj Vq ) can also provide a good first approximation to the second-derivative coupling matrix W( 2 ad(q>), which in a two-electronic-state approximation is given by... 

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