  Using a first-principles approach, we analyze the impact of \textit{DX}
centers formed by S, Se, and Te dopant atoms on the thermal conductivity of
GaAs. Our results are in good agreement with experiments and unveil the physics
behind the drastically different effect of each kind of defect. We establish a
causal chain linking the electronic structure of the dopants to the thermal
conductivity of the bulk solid, a macroscopic transport coefficient.
Specifically, the presence of lone pairs leads to the formation of structurally
asymmetric \textit{DX} centers that cause resonant scattering of incident
phonons. The effect of such resonances is magnified when they affect the part
of the spectrum most relevant for thermal transport. We show that these
resonances are associated with localized vibrational modes in the perturbed
phonon spectrum. Finally, we illustrate the connection between flat adjacent
minima in the energy landscape and resonant phonon scattering through detailed
analyses of the energy landscape of the defective structures.
