RHEED_DIFF_2D is an open-source software capable of performing qualitative numerical simulations of changes to the intensity of RHEED oscillations in the function of the layer deposited, employing various models of scattering crystal potential for heteroepitaxial structures of bcc (001) films nucleated on a fcc (001) surface, including the possible existence of various diffuse scattering models through the layer parallel to the surface. This model can be used for interpreting data weakly dependent on the azimuth of the incident electron beam direction and for 2D monolayer-by-monolayer growth modes. The program does not use any external data sources. New version program summaryProgram title: RHEED_DIFF_2DCPC Library link to program files:https://doi.org/10.17632/83ht2kc8xt.1Licensing provisions: GNU General Public License 3Programming language used: C++Journal reference of previous version: Computer Physics Communications 185 (2014) 3001–3009Does the new version supersede the previous version?: No. It is a supplement to the previous version.Reasons for new versionResponding to users' feedback we present a practical procedure of construction of simulation program, which facilitates the calculation of changes to the intensity of RHEED oscillations in the function of the layer deposited for 2D monolayer-by-monolayer growth modes, employing various models of scattering crystal potential for heteroepitaxial structures of bcc (001) Fe thin layers nucleated on the fcc (001) Ag substrate. The forms of these potentials can be freely defined by the user. The proposed approach may be generalized and applied to other metallic bcc/fcc or fcc/bcc interfaces.Nature of problemBulk iron has a bcc structure at room temperature and ferromagnetic below 920 K. It has been noticed that Fe monolayers grown on nonmagnetic substrates show a variety of interesting properties depending on the size of the lattice misfit between the substrate and growing layers, and on the thickness of the deposited material [1,2]. The fundamental scientific problem in such research is to specify both interface type and growth mechanism for subsequent layers. Researchers and technologists manufacturing two-dimensional nanoscale systems frequently use RHEED measurements in the function of the deposited layers to control growth of samples at the atomistic level of accuracy.Experimental data indicate that the iron grows on the Ag(001) surface as a bcc α-Fe film rotated by 450 relative to the underlying lattice [3]. For this reason, the net point spacing in plane (001) of Ag surface is 2.889 Å while that of bulk Fe(001) is 2.8665 Å. This leads to a compressive in-plane misfit (aFe – aAg/√2)/(aAg/√2) ≈-0.78%. This slight mismatch causes that during nucleation and in the early stage of growth, Fe should grow in 2D monolayer-by-monolayer mode on the Ag(001) substrate.Solution methodRHEED intensities are calculated within the general framework described in Ref. [4] with the model of the scattering potential for heterostructures:(1)UCombined(z)=∑nUsubstrate(z)+∑n(Ulayer(z)+Uaddlayer(z)),where the Uaddlaer represents the diffuse scattering on the topmost layer.Summary of revisionsThe presented version RHEED_DIFF_2D of the computer program implements an iterative algorithm for calculating the intensity of the specularly reflected electron beam as a function of growing layers. The pseudocode listed below illustrates the main assumptions of this algorithm.for all growing layers for all coverages of the overlayer for all elements of the scattering matrix Calculating the elements of the scattering matrix for the selected model of the scattering potential of the overlayer end for Calculating the amplitude of the specularly reflected electron beam end forend for