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How did Epicurus’s atomism differ from Democritus’s atomism?

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Bronze bust of Epicurus with a full beard and draped clothing, displayed on a round inscribed base at the National Archaeological Museum of Naples. — Epicureanism Epicurus, bronze bust from a Greek original, c. 280–270 bce; in the Museo Archeologico Nazionale, Naples.

While Epicurus adopted the foundational atomism of Democritus, his philosophy introduced a critical divergence that shifted the system from a strictly deterministic model to an indeterministic one.1

The primary differences include:

Direction and Motion: Democritus posited that atoms moved in all directions through the void in a chaotic, vibrating fashion, eventually colliding to form worlds. In contrast, Epicurus postulated an absolute up-down direction in space, suggesting that atoms fall in roughly parallel paths.21
The "Swerve": To account for how atoms in parallel paths could collide, Epicurus introduced the concept of a spontaneous "swerve" (clinamen). This unpredictable deviation interrupts the necessary chain of cause and effect.31
Free Will: Epicurus utilized this doctrine of the swerve to explain the existence of free will, whereas Democritus’s system was a purely mechanical system governed by "necessity," leaving no room for intelligent cause or individual agency.21

By incorporating the swerve, Epicurus transformed the earlier, rigid materialism of Democritus into a philosophy that allowed for human freedom within a universe composed entirely of atoms and void.31