  Based on first-principles calculations, we predict a substantial increase in
the optical dielectric function of LiAsSe$_2$ under pressure. We find that the
optical dielectric constant is enhanced threefold under volume compression.
This enhancement is mainly due to the dimerization strength reduction of the
one-dimensional (1D) As--Se chains in LiAsSe$_2$, which significantly alters
the wavefunction phase mismatch between two neighboring chains and changes the
transition intensity. By developing a tight-binding model of the interacting 1D
chains, the essential features of the low-energy electronic structure of
LiAsSe$_2$ are captured. Our findings are important for understanding the
fundamental physics of LiAsSe$_2$ and provide a feasible way to enhance the
material optical response that can be applied to light harvesting for energy
applications.
