I've found a few surprises in the code, though I haven't personally tested any of these. Attraction is calculated as detailed in the previous pages and then a few more tweaks are made.

• Beauty Wish - If either sim wished for Beauty from the genie and the other sim has an appropriate gender preference, then attraction is set to +100 and everything else is ignored. This could actually cause sims to have lower chemistry then they would have without the wish.
• Relationship - If sims have negative attraction but are married, engaged, going steady, in love, crushing or have LTR of at least 70 then attraction is set to 0 before continuing. Note this can cause a sudden jump in attraction from -1 bolts to +2 if one sim has one of the attraction benefits below.
• Twikkii Island - Being on Twikki increases attraction by a whopping +2.
• Benefits - The vacation benefit Je Ne Sais Quoi and the Romance benefit Massive Attraction each increase attraction by +40. These stack with each other and across both sims so if both sims have both benefits that's a +160 bonus over everything else.
• Potions - Either the gypsy's Love Potion or the Beauty Cocktail that can be made from the juicer adjusts attraction by +50 for a successful potion and -50 for an unsuccessful one. They do not stack so drinking both a Love Potion and a Beauty Cocktail has no additional effect.
• Repulsion Factor - Although the total attraction is an average of individual attractions, if either individual sim has an attraction toward the other of <-25 AND the averaged attraction is > 0, then the final averaged attraction score will be adjusted by -50. This would commonly be due to a failed love potion or a turn off.

The following things do NOT affect chemistry: interests, level of gender preference (appropriate gender preference is necessary, but a preference of +1 is just as attractive as +1000), secondary aspirations (only primary count). Relationships (crush, love, friends, best friends, engagement, marriage) do not directly affect chemistry but do set a minimum attraction.