Weyl semimetals are new states of matter which feature novel Fermi arcs and exotic transport phenomena. Based on first-principles calculations, we report that the chalcopyrites CuTlSe2, AgTlTe2, AuTlTe2 and ZnPbAs2 are ideal Weyl semimetals, having largely separated Weyl points (∼ 0.05Å −1 ) and uncovered Fermi arcs that are amenable to experimental detections. We also construct a minimal effective model to capture the low-energy physics of this class of Weyl semimetals. Our discovery is a major step toward a perfect playground of intriguing Weyl semimetals and potential applications for low-power and high-speed electronics.Weyl fermions, originally introduced as massless chiral fermions, are described by the Weyl equation [1]. Even though a number of elementary particles were considered as candidates of Weyl fermions, conclusive evidences of Weyl fermions as elementary particles are still lacking. Weyl fermions were also proposed as emergent low-energy quasiparticles in condensed matter systems breaking either time-reversal or spatial-inversion symmetry [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. One hallmark of Weyl semimetals is the existence of Fermi arcs in surface states [3]. So far the only experimentally known Weyl semimetals are the TaAs-class compounds, in which two sets of inequivalent Weyl points away from the Fermi level and complex Fermi surfaces were found by ARPES experiments [17][18][19]. Experimental evidences of negative magnetoresistance [2,20] induced by the chiral anomaly were also reported [21][22][23][24][25][26]. However, definite signatures of the chiral anomaly in the quantum limit, such as the linear-B negative magnetoresistance [2,20,27] and the emergent supersymmetry [28], and others [29][30][31][32] haven't been experimentally observed in known Weyl semimetals, which is partly due to the facts that the Weyl points are not all at the Fermi level and that there are coexisting trivial Fermi pockets. Therefore, it is urgent to discover ideal Weyl semimetals with only symmetry-related Weyl points at the Fermi level.In this work, we focus on a large family of ternary chalcopyrites ABC 2 at stoichiometry, which were of great interest because of potential applications including the thermoelectric effect, non-linear optics and solar cells [33,34]. Recently, some ternary chalcopyrites were predicted to be topological insulators [35]. Here, our first-principles calculations find that the chalcopyrite compounds CuTlSe 2 , AgTlTe 2 , AuTlTe 2 and ZnPbAs 2 are a class of ideal Weyl semimetals having eight symmetry-related Weyl points exactly at the Fermi level, but without any fine tuning. CuTlTe 2 and ZnPbSb 2 are also Weyl semimetals having eight symmetry-related Weyl points in energy gaps but have a few coexisting trivial bands around the X point; consequently they are not ideal Weyl semimetals at stoichiometry but can be tuned to be ideal Weyl semimetals by gating or doping. The ideal Weyl semimetals predicted in the chalcopyrites are analogous to those in compressively strained Hg...