Higher-order effective Hamiltonian for light atomic systems

We present the derivation of the effective higher-order Hamiltonian, which gives $m \alpha^6$ contribution to energy levels of an arbitrary light atom. The derivation is based on the Foldy-Wouthuysen transformation of the one-particle Dirac Hamiltonian followed by perturbative expansion of the many particle Green function. The obtained results can be used for the high precision calculation of relativistic effects in atomic systems.