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On the relation between the scalar and tensor exchange-correlation kernels of the time-dependent density-functional theory

The scalar $f_{xc}$ and tensor $\hat{f}_{xc}$ exchange-correlation (xc) kernels are key ingredients of the time-dependent density functional theory and the time-dependent current density functional theory, respectively. We derive a comparatively simple relation between these two kernels under the assumption that the dynamic xc can be considered "weak". We expect our formula to serve as a convenient bridge between the scalar $f_{xc}$ which directly enters many applications and the tensor $\hat{f}_{xc}$ which, due to its locality in space, is much easier to approximate.