A hybrid functional is defined as an approximate KS density functional where a part or all the semi-local DFT exchange expression 𝐸 𝑋 𝐷𝐹𝑇 is replaced by exact Hartree-Fock (HF) exchange 𝐸 𝑋 𝐻𝐹 . The amount of HF exchange for typical hybrid functionals lies in the range of 10-25% but can be as high as 50-55% like in the BHLYP and the M06-2X functional. Increasing the amount of Hartree-Fock exchange also increases the likelihood of encountering triplet instabilities (i.e., imaginary triplet excitation energies which indicate that the ground state is unstable with respect to symmetry breaking. Moreover, Incorporation of exact Hartree-Fock exchange reduce the self-interaction error (SIE) which is a significant trouble-maker in TD-DFT. However, it also introduces a four-index integral into the Hamiltonian which leads to higher computational cost in comparison to the GGA and meta-GGA functionals.