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Second-order Rayleigh-Schrodinger Hartree-Fock perturbation theory is applied to evaluate the correlation energy for the 4fI4 shell of Yb. The partial-wave (PW) increments to the pair energies, and the total correlation energy are calculated using first-order radial pair functions obtained as the solution of twodimensional differential equations. These equations are solved using the Chalmers University version of the finite difference method. The total second-order correlation energy amounts to 0.89573 hartree. The results for the pair energies, which are close to the all-external pair energies, are compared with related CI values. It turns out that the correlation effects within the 4f shell of rare-earth systems are dominated by the all-external effects. The ff and gg PWS are dominant in the description of the correlation effects in the 4f shell and provide 80% of the total correlation energy. Next in importance are the dd, hh, and dg PWS which provide 12% of the total correlation energy.

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The pair correlation energies for some n14f pairs of the ground state of the Yb atom are calculated for the first time. The partial wave (PW) increments to the second-order pair energies are generated using numerical first-order radial pair functions obtained as the solution of two-dimensional differential equations. The analysis of the PWS contributions shows the dominant role of the df, fg, and gh PWS for the 4d4f pair, of the pf and dg PWS for the 4p4f and 5p4f pairs, and of the sf and p g PWS for the 4s4f, 5s4f, and 6s4f pairs. A discussion of the similarities and differences of the structure of the correlation energy found in this paper with those calculated earlier for smaller atoms is given.

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