  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.
