After thermoelectric characterization, the DIL packages were connected to the room temperature sample holder of a 8404 Hall measurement system (LakeShore). Using two MFLI lock-in amplifiers (Zurich Instruments AG) and a CS580 voltage driven current Source (Stanford Research Systems) the double AC Hall (DAC) method 7 was applied to significantly increase the signal to noise ratio. Our setup is verified by DC reference measurements on InAs. Here, the frequency of the magnetic field f B = 0.8 Hz and the frequency of the current excitation was set to f I = 77 Hz. For different excitation currents I ex and different magnetic field B up to 0.3 T the Hall voltage FIG. . Calculated Seebeck coefficient of CuP2 as a function of temperature. The Seebeck coefficient is determined by semiclassical Boltzmann transport theory using the BoltzTraP2 package 11 on the CuP2 band structure. We employed the band structure calculated by density functional theory under the generalized gradient approximation (GGA) and available on the Materials Project database (mp-927). The p-type Seebeck coefficient is calculated at different hole concentrations, indicated by colors. A hole relaxation time of 10 fs is assumed. We further assume that the increase in conductivity of the experimental CuP2-y sample with temperature (Fig. (a), main article) is due to increasing hole concentration, which is in the 10 16 cm -3 range at room temperature (Fig. , main article). Under these assumptions, the arrow from 1 × 10 16 cm -3 at 200 K to 9 × 10 16 cm -3 at 300 K roughly indicates the expected temperature dependence of the Seebeck coefficient for a p-type CuP2 crystal. The transport calculations include the possible influence of other valence band maxima energetically close to the absolute valence band maximum along the Γ-Y line.