An accurate determination of the effective electric field (E eff ) 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 E eff 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.The electric dipole moment (EDM) of a nondegenerate system arises from violations of both the parity (P) and the time-reversal (T) symmetries [1]. T violation implies charge parity (CP) violation via CPT theorem [2]. In general, CP violation is a necessary condition for the existence of the EDMs of physical systems, and, in particular, atoms and molecules. Paramagnetic atoms and molecules are sensitive to the EDM of the electron (eEDM) [3], which is an important probe of the physics beyond the standard model [4]. The eEDM arising from CP violation could also be related to the matter-antimatter asymmetry in the universe [5]. A number of studies using atoms have been performed during the past few decades to extract an upper limit for the eEDM [6]. In general, for heavy polar molecules, the effective electric field experienced by an electron (E eff ) obtained from relativistic molecular calculations can be several orders of magnitude larger than that in atoms [7]. Therefore, the experimental observable (i.e., the shift in energy because of the interaction of the electric field with the eEDM) is also several orders of magnitude larger. Owing to the high sensitivity of the eEDM in molecules, there has been a considerable increase in interest in this field during the past decade The aim of the present work is to calculate E eff in YbF using a rigorous relativistic many-body method, which is more accurate than the methods used in the previous calculations. The method we have chosen is the four-component relativistic coupled-cluster (RCC) method, which is arguably the current gold standard for calculating the electronic structure of heavy atoms and diatomic molecules [18].The electron EDM interaction Hamiltonian in a molecule can be written as [19] Here, d e is the eEDM of an electron, is one of the Dirac matrices, and are the Pauli spin matrices. i is the index of summation labelling for electrons and N e is the total number of electrons. E int is the electric field acting on an electron in a molecule. The quantity that is of experimental interest in the search for the eEDM is an energy shift (E) of a particular state owing to the interaction Hamiltonian given in Eq.(1). This can be expressed as