After completing the first experiment for the good of humanity, Overseer Barstow requests the Sole Survivor perform another experiment, one that involves returning above ground to secure chemical formulas from HalluciGen, Inc. to distributed through a soda fountain.
Navigate through the entire facility to get to the basement research facility, and grab the Vault 88 chemical research in the steamer trunk.
Return Overseer Barstow.
Build the soda fountain prototype, power it up and connect it to a terminal in order to select an experimental parameter. It does not matter which parameter is chosen.
After Clem has served drinks for an hour, talk to him, and then report to Overseer Barstow.