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 on a Sim's picture in the relationship panel), certain social interactions will be made available earlier in the relationship, and it will be 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-OffsEdit
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/offsEdit
- Black, Blond, Brown, Red, Grey, Custom, Facial Hair
- Swim Wear, Underwear, Formal Wear
- Glasses, Hats, Jewelry (Must have Bon Voyage or newer EP)
- Makeup, Full face make-up
- Fat, Fit
- Cologne, Stink
- Life state
- Lycanthropy, Plantsimism, Zombiism, Robots (Must have Bon Voyage or newer EP)
- Good at Skill (Must have Bon Voyage or newer EP)
- Cooking, Cleaning, Creativity, Body, Logic, Charisma, Mechanical
- Employment (Must have Bon Voyage or newer EP)
- Unemployed, Hard Worker
- "Hard Worker" comes into effect if a Sim is at level 6 or higher in his or her career track.
- The "Good at Skill" turn-ons/turn-offs come into effect if a Sim's level in that skill is higher than 5.0, even if it has not yet reached 6. Using SimPE to examine Sims with skill levels between 5.0 and 6 shows that the chemistry trait for a skill is set when the skill level passes 5.0.
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. 
|Averaged Attraction Score||Chemistry|
|−25 or less||−1 bolt (red X)|
|−24 to 0||0 bolts (nothing)|
|1 to 34||1 bolt|
|35 - 89||2 bolts|
|90 or greater||3 bolts|
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:
|17.5||Wealth-Popularity, Wealth-Family, Popularity-Pleasure, Family-Grilled Cheese, Pleasure-Grilled Cheese|
|0||Romance-Wealth, Romance-Popularity, Wealth-Knowledge, Knowledge-Grilled Cheese|
|−17.5||Romance-Knowledge, Romance-Grilled Cheese, Wealth-Pleasure, Popularity-Family, Popularity-Knowledge, Family-Pleasure|
|−35||Romance-Family, Wealth-Grilled Cheese, Popularity-Grilled Cheese, Knowledge-Pleasure|
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 effect on the game.
Sims with the same zodiac sign have a zero attraction score. Most pairings of zodiac signs have a zero attraction score, so to avoid clutter, any sign pairing not listed in the following table has a zero attraction score.
|35||Aries-Taurus, Gemini-Pisces, Scorpio-Pisces, Capricorn-Aquarius|
|17.5||Taurus-Libra, Taurus-Capricorn, Gemini-Virgo, Cancer-Leo, Cancer-Libra, Cancer-Scorpio, Leo-Scorpio, Leo-Sagittarius, Virgo-Libra, Virgo-Sagittarius, Sagittarius-Capricorn, Sagittarius-Aquarius, Sagittarius-Pisces|
|−17.5||Aries-Libra, Aries-Pisces, Gemini-Cancer, Gemini-Leo, Leo-Virgo, Leo-Pisces, Libra-Sagittarius, Libra-Pisces, Scorpio-Sagittarius|
|−35||Aries-Cancer, Taurus-Virgo, Gemini-Capricorn, Leo-Capricorn, Libra-Scorpio, Scorpio-Aquarius|
Again, note that some of these zodiac pairings are one-sided, but the source cited decided to average them out.
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.
|0||+7||0 or 10||-7|
|1||+6||1 or 9||-5|
|2||+5||2 or 8||-4|
|3||+3||3 or 7||-2|
|4||+2||4 or 6||-1|
Turn Ons and Turn OffsEdit
Turn-ons and turn-offs are the part of chemistry that will easily distinguish which traits Sims like and don't like. Each trait that matches Sims' turn-ons adds a +17.5 to the attraction score and -22.5 for a matching turn-off. Here are some notes about the expanded turn-ons and turn-offs introduced in Bon Voyage:
- Wearing glasses or jewelry will set the Lycanthropy trait.
- Servos are set with random hair color.
- When selecting a custom hair, one of the hair colors will be highlighted. The highlighted hair is the actual hair color trait that's set. Thus, Sims can actually never have the custom hair trait.
- Wearing glasses will set the jewelry trait, but wearing jewelry will not set the glasses trait.
- Having a stubble or beard will not set glasses, jewelry, makeup, and full-face makeup by changing appearance in mirror. It can be set by removing the facial hair, changing any of the four appearances, and then setting the facial hair again.
- PlantSims and Witches will always have full-face makeup set.
Some of the expanded turn-ons and turn-offs have been fixed by Tunaisafish's Attraction Traits Fix.
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 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, which will add attraction points by +50.
A Sim's chemistry with everyone can be temporarily modified by ±50 attraction points by using the love potion.
- Do not attempt to move out Sims whose wish for beauty is still in effect. Doing so will cause the attraction boost to be permanent.