Higher-order many-body perturbation-theory calculations of energy levels in cesium

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Abstract
The question of which contributions to the energy levels of cesium must be included in many-body perturbation theory to achieve accuracies of parts per thousand is addressed. To this end, we evaluate second- and third-order correlation corrections to the energy levels of several states. When these corrections are supplemented with two dominant classes of corrections from fourth and higher order, the calculated energy of each of the states considered is brought to within a few parts per thousand of the measured energy. Implications for calculations of transition amplitudes, in particular, amplitudes of parity-nonconserving transitions, are discussed.