With the aim to benchmark the accuracy and computational performance of the DF-NEO-MP2 implementation, we compare the results to an integral-direct NEO-MP2 implementation and generally compare the achieved accuracy with previously reported density fitting approximations for NEO methods. For that reason we employ size increasing protonated water clusters as trial systems. These are well established reference systems for multicomponent methods. Preliminary error analysis of di↵erent density fitting methods within the NEO framework was already carried out by the group of Hammes-Schi↵er for the density fitting of NEO specific types of integrals (electron-electron, electron-proton, proton-proton) within the DF-NEO-CCSD method, in general for the accuracy of the DF-NEO-MP2 determining proton a nities and for the absolute deviation of the water energy with one and two quantum protons of the DF-NEO-OOMP2 method. However, the overall scaling of the density fitting error with respect to increasing system sizes and the achieved computational performance remains elusive. Therefore, we computed the absolute deviation of the obtained energy from DF-NEO-MP2 obtained with the cc-pVTZ-MP2FIT and cc-pVQZ-MP2FIT density fitting basis sets to a complete integral-direct NEO-MP2 version and compared also the computational e↵ort of these methods for the size increasing protonated water clusters. The results are shown in Fig. . By increasing the fitting basis set size it is possible to reduce the error from 0.02 0.06 kcal mol 1 for the cc-pVTZ-MP2FIT set down to 0.01 0.03 kcal mol 1 for the cc-pVQZ-MP2FIT set. The overall error with respect to the increasing system size scales rather linearly and the obtained errors induced by the density fitting approximation are similar to those observed by Hammes-Schi↵er and coworkers. Regarding the computational savings by employing the density fitting approximation the speed-ups range between factors of 5 28 for the cc-pVTZ-MP2FIT set and only slightly reduced to a factor of 4 22 for the cc-pVQZ-MP2FIT set. The achieved computational speed-up is especially notable since the overall error can be reduced by half when employing the larger cc-pVQZ-MP2FIT and cc-pVTZ-MP2FIT