A Meeting of the Minds

 is a Fallout 3 quest and achievement that is added to the game with the Point Lookout add-on.

Ending the feud
After telling Desmond that you are ready to go, you will both make your way to the Lighthouse. Once you make your way to the base of the Lighthouse, enter it. When you enter, a section of the Lighthouse floor slides away and reveals a door to what is called the “Underground Lab”. In the lab, you will make your way to a room with a closed metal door. Destroy or disable the turrets that begin to attack you and make your way through the hallway behind the closed door. As you make your way through the hallways, look in the rooms and get supplies that are in the ammo boxes.

Who to kill?
When you find the brain you must talk to him and decide what you’re going to do. If you decide to kill Desmond, it is an easy fight for a high-level character. Make your way through the complex and talk to Professor Calvert, who will then reward you... with death. Fight him by shooting his glass tank, and eventually it will shatter. The Protectrons he sends after you should be a minor hindrance at best. If you choose to kill Calvert, then the battle will unfold in the same way.

Once the fighting is finished, go to the door in the information room, and inside you will find a workbench covered with lots of Energy ammo, a Microwave Emitter, two ammo boxes, and a safe with some money and other random stuff in it.

Killing Desmond
If you decide to kill Desmond, the Professor will reward you with the best thing any human could ever hope for... death. Or at least he'll try, at which point he'll send the robots after you. Destroy them and loot Desmond's body to find his unique glasses (+5 to small guns and explosives).

Killing Calvert
If you don't want to kill Desmond, shoot at the brain until the glass shatters and the water completely drains, then shoot the brain itself. After the brain is dead talk to Desmond, he will give you a key which unlocks the door which requires a key nearby. Inside this door is a cache of energy weapon ammo and weapon components, as well as the Microwave Emitter. And if you feel like being dirty and don't really care about Desmond, you can kill him afterwards and claim his Eyeglasses. He also carries a Combat Shotgun and a Sniper Rifle. Killing Calvert does not make the tribals at the Cathedral hostile to you, but if you chose to side with Desmond in Thought Control they will be already.

Note: you can also get Desmond's Eyeglasses by using the theft trick, just plant a helmet on him that can't go with glasses, (ie.power helmet) exit the mansion, save the game then load the game, re-enter the mansion, and Desmond should be wearing the helmet. then pickpocket it from him