  The operation of Terahertz (THz) communication can be significantly impacted
by the interaction between the transmitted wave and the molecules in the
atmosphere. In particular, it has been observed experimentally that the signal
undergoes not only molecular absorption, but also molecular re-radiation. Two
extreme modeling assumptions are prevalent in the literature, where the
re-radiated energy is modeled in the first as additive Gaussian noise and in
the second as a scattered component strongly correlated to the actual signal.
Since the exact characterization is still an open problem, we provide in this
paper the first comparative study of the performance of a reconfigurable
intelligent surface (RIS) assisted THz system under these two extreme models of
re-radiation. In particular, we employ an RIS to overcome the large pathloss by
creating a virtual line-of-sight (LOS) path. We then develop an optimization
framework for this setup and utilize the block-coordinate descent (BCD) method
to iteratively optimize both RIS configuration vector and receive beamforming
weight resulting in significant throughput gains for the user of interest
compared to random RIS configurations. As expected, our results reveal that
better throughput is achieved under the scattering assumption for the molecular
re-radiation than the noise assumption.
