The adiabatic potential energy surface which connected the triplet self-trapped exciton state to third-nearest neighbor Frenkel pairs was calculated for Na18F4 clusters by using an all electron  ab initio  Hartree-Fock approach. The correlation effect revealed more details, such as the positions of the nearest-neighbor Frenkel pairs, but did not appreciably alter the results of unrestricted Hartree-Fock calculations. No potential barrier that was higher than 0.2eV was observed up to third-nearest neighbor separation. It was concluded that the self-trapped excitons could evolve dynamically into stable Frenkel pairs.

K.S.Song: Physical Review B, 1996, 53[19], 12537-40