Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).While the role of sampling of the electron momentum k in supercell calculations of the elastic electron transmission is well understood, its influence in the case of inelastic electron tunneling (IET) has not yet been systematically explored. Here we compare ab initio IET spectra of molecular monolayers in the commonly used Γ-point approximation to rigorously k-converged results. We study four idealized molecular junctions with either alkanedithiolates or benzenedithiolates, and explore variations due to varying molecular tilt angle, density, as well as chemical identity of the monolayer. We show that the Γ-point approximation is reasonable for a range of systems, but that a rigorous convergence is needed for accurate signal amplitudes. We also describe an approximative scheme which reduces the computational cost of the k-averaged calculation in our implementation.We acknowledge the support of the Basque Departamento de Educacion and the UPV/EHU (Grant No. IT-756-13), the Spanish Ministerio de Economía y Competitividad (MINECO Grants No. MAT2013-46593-C6-2-P and FIS2013-48286-C2-2-P), and the European Union Integrated Project PAMS (Contract No. 610446).Peer Reviewe
