  In a recent paper it has been shown that to model a diffusive molecular
communication (MC) channel with multiple fully absorbing (FA) receivers, these
can be interpreted as sources of negative particles from the other receivers'
perspective. The barycenter point is introduced as the best position where to
place the negative sources. The barycenter is obtained from the spatial mean of
the molecules impinging on the surface of each FA receiver. This paper derives
an expression that captures the position of the barycenter in a diffusive MC
channel with multiple FA receivers. In this work, an analytical model inspired
by Newton's law of gravitation is found to describe the barycenter, and the
result is compared with particle-based simulation (PBS) data. Since the
barycenter depends on the distance between the transmitter and receiver and the
observation time, the condition that the barycenter can be assumed to be at the
center of the receiver is discussed. This assumption simplifies further
modeling of any diffusive MC system containing multiple FA receivers. The
resulting position of the barycenter is used in channel models to calculate the
cumulative number of absorbed molecules and it has been verified with PBS data
in a variety of scenarios.
