  We study the accuracy of the divide-and-conquer method for electronic
structure calculations. The analysis is conducted for a prototypical subdomain
problem in the method. We prove that the pointwise difference between electron
densities of the global system and the subsystem decays exponentially as a
function of the distance away from the boundary of the subsystem, under the gap
assumption of both the global system and the subsystem. We show that gap
assumption is crucial for the accuracy of the divide-and-conquer method by
numerical examples. In particular, we show examples with the loss of accuracy
when the gap assumption of the subsystem is invalid.
