0_7347 In excitonic insulator candidates, like 1T-TiSe2 and Ta2NiSe5, the simultaneous presence of structural distortions and strong Coulomb interaction prevents a transparent classification of the driving mechanism for the symmetry-breaking transition. I will present how collective response can be employed to classify the importance of each degree of freedom and gauge them with experimental data.I will focus on the collective mode dynamics in realistic models of Ta2NiSe5, which leads to much richer physics than typically anticipated from minimal models. The modelling explains how the amplitude and the phase modes are realized in the material and what are its symmetry properties. Considering the realistic band structure, I will show that the mass of the phase mode can reveal how the electronic symmetry is
reduced in the material. Remarkably, the collective mode analysis shows that previously reported phases are not in the ground state unless the system exhibits strong coupling with lattice degrees of freedom. These symmetry arguments give firm boundaries on the competition between microscopic degrees of freedom and we propose non-linear optical processes which are directly sensitive to the collective response. The study will explain how a combination of realistic modelling and symmetry arguments is a generic tool for understanding the rich collective mode structure, and it should be employed in other excitonic insulator candidates.