  We introduce an overlapping-domain approach to large-area metasurface design,
in which each simulated domain consists of a unit cell and overlapping regions
from the neighboring cells plus PML absorbers. We show that our approach
generates greatly improved metalens quality compared to designs produced using
a locally periodic approximation, thanks to $\sim 10\times$ better accuracy
with similar computational cost. We use the new approach with topology
optimization to design large-area ($200\lambda$) high-NA (0.71) multichrome and
broadband achromatic lenses with high focusing efficiency ($\sim 50\%$),
greatly improving upon previously reported works.
