  The GW method, which can describe accurately electronic excitations, is one
of the most widely used ab initio electronic structure technique and allows the
physics of both molecular and condensed phase materials to be studied. However,
the applications of the GW method to large systems require supercomputers and
highly parallelized software to overcome the high computational complexity of
the method scaling as $O(N^4)$. Here, we develop efficient massively-parallel
GW software for the plane-wave basis set by revisiting the standard GW formulae
in order to discern the optimal approaches for each phase of the GW calculation
for massively parallel computation. These best numerical practices are
implemented into the OpenAtom software which is written on top of charm++
parallel framework. We then evaluate the performance of our new software using
range of system sizes. Our GW software shows significantly improved parallel
scaling compared to publically available GW software on the Mira and Blue
Waters supercomputers, two of largest most powerful platforms in the world.
