What do you do when two hands are not enough? Interactive selection of bonds between pairs of tangible molecules

For molecular modeling, chemical structures have to be understood and imagined both in their three-dimensional spatial extent and in their dynamic behavior. We have developed an AR-based system for tangible interaction with molecules using optical markers. When users bring several molecules close to one another, potential bonds are shown and the molecules dynamically change their 3D structure according to potential chemical reactions. A problem arises when users also need to select one such bond from of a multitude of potential bonds while already using both hands to manipulate the molecules. We present two gesture-based techniques, shake-based and proximity-based to solve this problem. We report on user tests evaluating these techniques with respect to speed, precision and user acceptance.