**Abstract** Introducing tungsten (W) as a material for plasma-facing components in fusion devices brings new challenges for plasma simulation tools. High-Z impurities like W (Z=74) will not be fully ionized even in the core plasma of planned fusion devices like ITER and DEMO. Bound electrons strongly influence collisions in plasma by screening the nucleus in elastic collisions, but also by introducing new mechanism of energy loss by excitation and ionization [1, 2]. These additional losses can change the critical field needed for the creation of runaway electrons. It is possible to incorporate these effects into the Fokker-Planck equation by applying corrections to the collision frequency parameters, e.g. as presented by Hesslow et al [3]. Unfortunately, the precise calculations of the partial screening effect require parameters specific to the given atomic model. The use of ab-initio atomic models is computationally demanding, therefore it is necessary to introduce some semi-empirical atomic models. This entails significant approximations into calculations, so it is a tradeoff between accuracy and computation time. To solve this problem, a new semi-empirical model is proposed in the present work, based on the Pratt-Tseng model. It is numerically optimized using DFT ab-initio calculations as a reference. Such solution allows to obtain very short computation time, without sacrificing accuracy of the solution in the process. [4] **Acknowledgment** This work has been partially funded by the National Science Centre, Poland (NCN) grant HARMONIA 10 no. 2018/30/M/ST2/00799. We thank the PLGrid project for computational resources on the Prometheus cluster. This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No 101052200 — EUROfusion). Views and opinions expressed are however those of the authors only and do not necessarily reflect those of the European Union or the European Commission. Neither the European Union nor the European Commission can be held responsible for them. **References:** [1] A. Jardin et al, Institute of Nuclear Physics PAN, 2020, Report No 2105/AP. [2] Y. Peysson et al, *IAEA FEC 2020*, May 2021. [3] L. Hesslow et al, *Phys. Rev. Lett. 118* (2017) 255001. [4] J. Walkowiak et al, *Phys. Plasmas 29*, 022501 (2022).