  The all-order correlation potential method of accurate atomic structure
calculations for atoms with one external electron is extended to include one
more class of correlation diagrams to all orders. These are the so-called
ladder diagrams which describe residual Coulomb interaction between an external
electron and atomic core. This is in addition to the screening of Coulomb
interaction by core electrons and the hole-particle interaction in the core
polarization operator which are also included in all orders. Calculations of
the energies of the lowest $s$, $p$ and $d$ states of cesium and thallium show
that inclusion of the ladder diagrams leads to significant improvement of the
accuracy of the calculations. The discrepancy between theoretical and
experimental energies is reduced to a small fraction of a per cent in all
cases. This widens the range of atoms and atomic states for which the
correlation potential method can produce very accurate results.
