  An accurate determination of the effective electric field (Eeff) in YbF is
important, as it can be combined with the results of future experiments to give
an improved new limit for the electric dipole moment of the electron. We report
a relativistic coupled-cluster calculation of this quantity in which all the
core electrons were excited. It surpasses the approximations made in the
previous reported calculations. We obtain a value of 23.1 GV/cm for Eeff in YbF
with an estimated error of less than 10%. The crucial roles of the basis sets
and the core excitations in our work are discussed.
