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Chemistry is a game mechanic for The Sims 2, first introduced in the Nightlife expansion pack. Chemistry overhauled the in-game romantic attraction mechanic between Sims, making it easier or more difficult to foster romantic relationships between Sims.
Chemistry measures multiple aspects of a Sims' personality to determine how easily Sims can be attracted to one another. Certain pairings of aspirations can result in greater or lesser chemistry. Some zodiac signs share a strong attraction (or repulsion) to other signs. Similarity in Sim personality points can improve attraction. Sims possess turn-ons and a turn-off, which are specific traits that the Sim finds especially attractive or repulsive.
These factors are considered together, and the combined score determines the level of chemistry, demonstrated in-game by a number of "bolts" present in the relationships panel. The greater the number of bolts, the stronger the attraction between two Sims. Strongly attracted Sims are more easily able to form relationships, and are more likely to have positive interactions with each other.
A complex chemistry mechanic does not appear in The Sims or The Sims 3.