  We present a new method for solving the three-dimensional gravitational
potential of a density field on the Yin-Yang grid. Our algorithm is based on a
multipole decomposition and completely symmetric with respect to the two
Yin-Yang grid patches. It is particularly efficient on distributed-memory
machines with a large number of compute tasks, because the amount of data being
explicitly communicated is minimized. All operations are performed on the
original grid without the need for interpolating data onto an auxiliary
spherical mesh.
