Merits and Drawbacks of Direct Perturbation Theory

CONCLUSIONS. MERITS AND DRAWBACKS OF DIRECT PERTURBATION THEORY 

The direct perturbation theory (DPT) of relativistic effects has a few nice features. 

It can be formulated alternatively in terms of 4-component- or 2-component spinors, so it is directly related to both a fully relativistic theory and a quasi-relativistic theory. 

There is no problem of variational collapse or of a Brown-Ravenhall disease. The need for a no-pair projection never arises. 

Convergence is guaranteed, although it may become slow for high nuclear charges. 

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