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Nonvariational calculation of the relativistic and finite-size corrections for the helium ground state.

Relativistic and finite-size corrections are calculated by using an accurate direct solution of the Schroedinger equation [the correlation function hyperspherical harmonic (CFHH) method] for the ground state of the helium atom. In the CFHH method the solution is a product of a correlation function and of a smooth factor expanded into hyperspherical harmonic functions. Given the proper correlation function, chosen from physical considerations, the method generates wave functions, accurate in the whole range of interparticle distances, which leads in turn to precise estimates of the relativistic operators. Our nonvariational results are compared with those obtained in recent variational calculations.