Atomic is an auto parts company in Grand Theft Auto: San Andreas, Grand Theft Auto IV and Grand Theft Auto V.
GTA San Andreas
TransFender and Wheel Arch Angels offer Atomic brand custom rims. Wheel Arch Angels also offers an Atomic-themed paintjob for the Flash.
GTA IV
In GTA IV, they make all the tires in the game and some of the headlights.
GTA V
Atomic serves the same purpose as it did in GTA IV but now has its very own blimp in the sky advertising their products