The Watering Hole

The Watering Hole is a side quest in the Fallout 4 add-on Vault-Tec Workshop.

Detailed walkthrough
Overseer Barstow will tell you the next experiment requires a package from HalluciGen, Inc. which was never delivered due to the bombs.

The shipment is in the loot chest in the final room after navigating the building, and therefore is easier to use the back entrance facing the Charles View Amphitheater once the door is unlocked, if you have cleared it before.

Barstow will then reveal that the next vault productivity 'hazard' is hydration breaks. The notes reveal plans to introduce chemicals into the soda fountain you build, to different effects: Mood Enhancer, Appetite Suppressant, or Generic Caffeination. Using a terminal, select which parameters you wish to apply. Again, once you decide, you cannot change, and various responses from companions may occur.

Allow Clem to serve drinks from the fountain for an hour, and await the results from Barstow. Like before, they will reflect what the R&D team in each parameter description discovered. The mood enhancer option will also permanently affect Clem's mood, making him more positively-inclined towards everything.

Bugs
Attempting to talk to Clem after he has served drinks for an hour may produce a glitch, where Clem responds "You here to check up? I got a customer right here," but won't progress to the next stage of the quest. The female settler who arrived with Clem, the one with a hair bun, must be present at the soda fountain so she can talk to Clem before the quest will complete. Possible fix is to wait a few hours and/or leave the vault and re-enter, then she will be standing beside him.