Chemistry

Chemistry was introduced in the Nightlife expansion of The Sims 2. It is present in all expansion packs and stuff packs released after Nightlife. Until the introduction of chemistry, Sims were attracted to each other fairly easily. Only a massive divergence of interests would harm two Sims' relationships, but even that could be overcome with a bit of work.

Chemistry changes that. Although love can still blossom under any circumstances with enough effort, Sims now get two turn-ons and one turn-off from a limited list. If two Sims have positive chemistry (denoted by 1, 2, or 3 bolts under in the relationship panel), certain social interactions will be made available earlier in the relationship, and make it easier for all interactions to pass. Conversely, if two Sims have negative chemistry actions will fail more often. Sims on community lots can scope crowds to find those special Sims.

Family members who have strong chemistry with one another will want to be near one another. This can cause results such as a Sim overlooking a nearby kitchen sink in order to wash dishes in the bathroom, because a Sim they have chemistry with is in the bathroom. It is not possible to select chemistry based on gender, age, or skin color. However, it is possible to indirectly select a chemistry for gender by making the Sim like or dislike make-up and/or beards, and Sims can be made to dislike/like elder Sims by selecting grey hair. Gender preference also plays a role in chemistry. Once a Sim's preference for a gender becomes negative, the Sim will not have chemistry for members of that gender.

Turn-Ons and Turn-Offs
Each teen, adult and elder Sim has two turn-ons and one turn-off. Turn-ons and turn-offs are designed to make relationships easier (or harder) to achieve. Sims made before Nightlife get a potion called the ReNuYu Porta-Chug, which allows the Sim to change their turn-ons and turn-off, so they are not locked into randomly chosen ones. Turn-ons and turn-offs can also be changed by using the ReNuYuSenso Orb, which is an aspiration reward. If University is installed, college Sims can change their turn-ons and turn-off, as well as their aspiration, when they finish their Sophomore year. A Sim's turn-ons and turn-off can also be changed with SimPE. If changes to a Sim's turn-ons, turn-off, or aspiration would affect his or her chemistry with another Sim, the changes may not appear until the next time the two meet face-to-face.

List of Turn-on/offs

 * Hair
 * Black, Blond, Brown, Red, Grey, Custom, Facial Hair


 * Clothing
 * Swim Wear, Underwear, Formal Wear


 * Accessories
 * Glasses, Hats, Jewelry (Must have Bon Voyage or newer EP)


 * Makeup
 * Makeup, Full face make-up


 * Fitness
 * Fat, Fit


 * Smell
 * Cologne, Stink


 * Life state (Must have associated expansion pack)
 * Vampirism
 * Lycanthropy, Plantsimism, Zombiism, Robots (Must also have Bon Voyage or newer EP)
 * Witchiness (Must also have Apartment Life)


 * Good at Skill (Must have Bon Voyage or FreeTime)
 * Cooking, Cleaning, Creativity, Body, Logic, Charisma, Mechanical


 * Employment (Must have Bon Voyage or newer EP)
 * Unemployed, Hard Worker

Chimie

Calculating Chemistry
Turn-ons and Turn-offs are not the only contributors to Sim chemistry. Chemistry is determined based on the attraction score between two Sims. This attraction score is calculated from each Sim's zodiac, primary aspiration, current personality (not genetic), turn-ons, turn-offs, and any additional effects that affect attraction. All of these factors are added up to produce the attraction score and averaged to find the chemistry. Thus, one Sim may be more attracted to the other, yet both would have the same chemistry score. The actual calculations for these scores are slightly complicated, but easy to understand. 

Aspiration
Secondary aspirations do NOT count towards this score. Sims with the same aspiration have a +35 attraction score for that category with the exception of Grilled Cheese, which has a +145 score. Otherwise they are determined thusly:

According to Cyjon, some of these aspirations have a one-sided attraction, but he averaged them when he created his charts since he didn't find any real affect on the game.

Zodiac
Unlike Aspirations, Sims with the same zodiac have a zero attraction score. Most of the zodiacs have zero attraction, so to avoid too much clutter, any aspiration pairing NOT listed in the following table has a zero attraction score.

Again, note that some of these zodiac pairings are one-sided, but the source cited decided to average them out.

Personality
This whole section is the reason why the encouragement system is important. Sims with the same zodiac are neutral to each other, thus Sims that have the same exact genetic personality would get high scores here, but would never be beyond 70 points because their zodiacs cancel each other out. To get the amount of points needed for 3-bolt relationships you need to have a pair of signs that have +35 attraction and are close in personality. Thus it is far more likely that CAS Sims or playable Sims beyond the first generation that vary from the norm have 3 bolt relationships with their partners. The personality portion of the attraction score is the most complex easy to mess up.

For those of you who are math-oriented, here are the formulas used: If both qualities are on the same side of the scale (6-10 or 0-4) there is a bonus of 7 - (difference / 71). Note that 5 is excluded, although not when both sims have 5 points for that quality pair. If they are on opposite sides of the scale (say one is grouchy while the other is nice), there is a penalty of (difference between sim and 5)/71 for each sim added together.

If you don't especially care to track down every stray decimal, the following is a chart of estimated values based on personality points. Again, note that if the personality is on the same side of the scale, you use the first chart. If they are on opposite sides or one is 5, then use the second chart. When using the Penalty chart, you find the penalty for each sim and add them. If the combined penalty is less than -7, it becomes -7.

Turn Ons and Turn Offs
This section is very straight-forward when compared to the others. Each turn-on that is fulfilled adds a +17.5 to the attraction score and each turn-off adds a -22.5. If turn-offs and turn-ons don't follow this pattern, it means that they are broken. Tunaisafish's |Attraction Traits Fix helps, "though some bugs are buried too deep in the code to be moddable".

Other Influences
With FreeTime, Sims can wish for Beauty from the genie, which sets the attraction score to a flat +100 for all relationships he or she has with other Sims. This modification is actually set first and, if true, the rest of the calculations and effects are skipped. This means that if you wished for Beauty and then took a love potion, the love potion would be nulled until the Beauty wore off, which would result in a wasted love potion.

If two Sims with negative chemistry are married, engaged, going steady, in love, crushing, or develop a long term relationship of at least 70, their chemistry points will be increased to zero the next time they interact face-to-face. Although the total attraction is an average of individual attractions, if either individual Sim has an attraction toward the other of < -25, then the final averaged attraction score will be adjusted by -50. Note that both of these modifications are done after finding the Sims' chemistry from the methods above and is added onto by the the effects described below.

Also included in FreeTime, Sims whose primary or secondary aspiration is Romance can get the Massive Attraction aspiration benefit, which modifies the affected Sims' attraction points by +40. Bon Voyage has the vacation benefit Je Ne Sais Quoi, which also gives a +40, and being on Twikkii Island will also slightly increase a Sim's attractiveness by +2. Seasons has the Beauty Cocktail.

A Sim's chemistry with everyone can be temporarily modified by ±50 attraction points by using the love potion.