Free tDCS Modeling Tools
Courtesy of Danish Research Centre for Magnetic Resonance (DRCMR) and the Technical University of Denmark (DTU), is SimNIBS.
SimNIBS is a free and open source software package for the Simulation of Non-invasive Brain Stimulation. It allows for realistic calculations of the electric field induced by transcranial magnetic stimulation (TMS) and transcranial electric stimulation (TES)
Courtesy of Korea Brain Research Institute, Dongseo University, Hanyang University
Courtesy of the Neural Engineering Lab at CCNY, the ROAST, BONSIA and SPHERES free modeling tools are live and FREE. These modeling packages can be used to help design TES montages including for tDCS, tACS, tRNS, and more.
You can watch a detailed walkthrough of ROAST here. To get more detailed info on ROAST please visit CUNY Parra Lab.
Additional Details can be found by referencing this paper, The Point Spread Function of the Human Head and its Implications for Transcranial Current Stimulation