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How did Epicurus modify 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. — Materialism Epicurus, bronze bust from a Greek original, c. 280–270 bce; in the Museo Archeologico Nazionale, Naples.

Epicurus modified the atomism of Democritus by introducing an indeterministic element to his physical system. While Democritus envisioned atoms moving through space in various directions, Epicurus postulated an absolute up-down direction, suggesting that all atoms fall in roughly parallel paths.1

To account for the collisions between these atoms, Epicurus introduced the concept of the "swerve"—a spontaneous, chance deviation of atoms from their natural paths. This modification was significant for two primary reasons:12

Physics: It allowed for a mechanical explanation of how atoms interact and collide within the universe.12
Philosophy: It provided a basis for free will, as it interrupted the strictly necessary effects of causality, thereby avoiding a purely deterministic universe.12

By incorporating this "swerve," Epicurus transformed Democritus’s deterministic materialism into an indeterministic one, which he used to support his broader goal of quieting the fears of superstitious Greeks regarding natural laws and the nature of the gods.31