Second-harmonic generation (SHG) of CO2 lasers has been investigated since the early 1970s, resulting in the development of a number of efficient nonlinear crystals. Among the most suitable candidates for laser systems one finds are AgGaSe2, ZnGeP2, CdGeAs2, and AgGaS2. For these crystals measured conversion efficiencies of frequency doubling have reached levels of 35%, while basic principles and damage threshold of materials pose an upper limit well above that, at 50%. In the quest for optimal conversion we studied the SHG of a transferred-electron amplifier CO2 laser in an AgGaSe2 crystal, measuring a value of 20%, which is further scalable to 50%. In this paper we compare the various studies and layout the design parameters for an optimal second-harmonic generator for the 4.5- to 5.5-?m range.