Summary: We determine the ground state energy of the translation invariant Pauli-Fierz model for an electron with spin, to subleading order $O(\alpha^2)$ with respect to powers of the finestructure constant $\alpha$ and prove rigorous error bounds of order $O(\alpha^3)$. A main objective of our argument is its brevity.

81Q10, 35P15, 46N50, 47N50

translation invariant Pauli-Fierz Hamiltonian, spectral theory, ground state energy