Two-electron F' centers in alkali halides: a saddle point approach. I.   General and semicontinuum analyses
  The F' center in an alkali halide forms when an anion vacancy traps two electrons which is the prerequisite of a diatomic molecule. Indeed, the center may displace left or right along <110> in a (110) plane, due to its coupling to the B_{1u} vibrational mode of <110> polarization respectively. On jumping from the initial position to the final position the F' center passes through a saddle point which configuration is molecule-like being conformed by two neighboring semi-vacancies along <110>. Each semi-vacancy traps one electron to change its effective charge from +1/2 to -1/2 formin a semi-F' center. We ouline the basic theory so as to perform calculations on the F' eigenenergies in the two configurations to see which one of them is more favorable. 
Mladen Georgiev