I've found a few surprises in the code, though I haven't personally tested any of these. The actual routine goes in this order.

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.

Otherwise attraction is calculated as detailed in the previous pages and a few more tweaks are made.

If sims have negative attraction but are married, engaged, going steady, in love, crushing or have LTR of at least 70 then attraction is increased 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.

Being on Twikkii Island increases attraction by a whopping +2.

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.

The Love Potion adjusts attraction by +50 for a successful Love Potion and -50 for an unsuccessful one.

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.