In large light harvesting systems, not all pigments are coupled strongly. This is evidenced by the formation of delocalized states in certain domains of strongly coupled pigments. The threshold value for assigning pigments to domains is usually defined, and the pigment pairs in which the electronic coupling is greater than this value are included in the same domain to describe the dynamic localization effect implicitly. However, domain assignment by a single threshold value may make it difficult to include the possible localization of exciton states by temperature and the difference in the electronic excitation energy between pigments. In this study, we use the variational polaron approach for domain assignment to include such possible localization. To demonstrate the validity of domain assignment by the variational approach, we applied it to pigments in photosystem II (PSII) and compared the domain model constructed by the single threshold value. We showed that domain assignment by the variational approach could be used to determine the valid domain model in PSII without using the empirical threshold value at least at 77 K