[en] Highlights: • Dissociative isomerization pathways of dichloroethene cation/anion were automatically explored. • A geometry optimization in the ROCBS-QB3 method was employed in the exploration. • DFT energies for dichloroethene anion were compared with the ROCBS-QB3 results. Isomerization and dissociation pathways after the photoionization or electron attachment of (E)-1,2-dichloroethene were calculated with an automated exploration method utilizing a scaled hypersphere search of the anharmonic downward distortion following algorithm at the UB3LYP/6-311G(2d,d,p) level of theory. The potential energies of transition states and dissociation channels were calculated by a composite method ((RO)CBS-QB3) and compared with the breakdown diagrams and electron attachment spectra observed in previous spectroscopic studies. The results of single point calculations with several DFT and post-SCF methods are compared using the root mean square deviations from the (RO)CBS-QB3 energies for six states of anionic dichloroethene.