  We have previously used inverse statistical-mechanical methods to optimize
isotropic pair interactions with multiple extrema to yield low-coordinated
crystal classical ground states (e.g., honeycomb and diamond structures) in
d-dimensional Euclidean space R^d. Here we demonstrate the counterintuitive
result that no extrema are required to produce such low-coordinated classical
ground states. Specifically, we show that monotonic convex pair potentials can
be optimized to yield classical ground states that are the square and honeycomb
crystals in R^2 over a non-zero number density range. Such interactions may be
feasible to achieve experimentally using colloids and polymers.
