Hako no Naka no Ashita

Tomorrow in the Box is a three-way Science Adventure Series crossover novella featuring characters from ChäoS;HEAd, Steins;Gate, and Robotics;Notes. It was originally released on January 24, 2014 as part of the "SCIENCE ADV SERIES 5 Years Jubilee" book, which commemorated the series' fifth anniversary. Source: Steins;Gate Wiki