  The terahertz (THz) frequency band has recently attracted considerable
attention in wireless communications as potential candidate for providing the
necessary high bandwidth for demanding applications. With increasing frequency,
however, the communication link becomes more vulnerable to blockage and
pathloss increases. While both effects can be mitigated with the judicious
utilization of directional beams and Reconfigurable Intelligent Surfaces
(RISs), high directivity could potentially increase the probability of
undesired misalignment between the beam that is steered by the RIS, and the
user. It is therefore crucial to characterize and understand the stochastic
behavior of misalignment in RIS-aided THz links. In this work, beam
misalignment in RIS-aided links is studied theoretically and analytical models
are derived, the validity of which is verified through numerical calculations.
It is demonstrated that there is a distinction in the stochastic behavior of
misalignment between pointing errors that occur on the steering plane or
normally to the steering plane, with direct consequences on the link robustness
on misalignment. The analytical models capture the impact of misalignment under
these qualitatively different conditions and provide the necessary tools for
assessing the stochastic RIS performance with respect to crucial link
parameters, such as the transmitter's beam width, the transmitter-RIS distance,
the RIS-receiver distance, and the steering angle of the RIS.
