In dating sites, the word chemistry is a common marketing tool used by many sites, in an often unsubstantiated way, to claim that their matching algorithms can predict successful chemistry, either romantic, sexual, physical, etc., in relationships and as such pair people accordingly. [1] The matching site LoveIsSexy, for instance, boldly declares, on its about us page, that it has a “state-of-the-art matching technology” that puts its users in direct contact with “relationships that work” and that: [2]

“Relationship chemistry is more than simply liking the same movies … it’s about shared values, ethics, social, physical, and intellectual characteristics … it’s also about sex … without good sexual chemistry and compatibility, many relationships simply don’t work.”

These claims and various terminologies, however, from a scientific point of view are unsubstantiated popularizations of what is, in reality, a set of very difficult subjects (human chemistry + human thermodynamics) in infancy. [3] Other sites that openly claim to be able to match according to chemistry include: eChemistry (2003), Chemistry (2006), ScientificMatch (2007), among others.

To cite one metaphorical formulation of the word "chemistry" in relationship terms, American author Andrea Orr, in her 2003 book Meeting, Mating, and Cheating: Sex, Love, and the New World of Online Dating, gives the following expression: [4]

Love – Chemistry = Friendship

In rigorous scientific terms, however, realistic equations to define "chemistry" occurring between people was outlined via affinity reactions by German polymath Johann von Goethe in 1809. In modern terms, affinity is defined via chemical thermodynamics, as to are human chemical reactions.

References
1. (a) Chemistry, (b) Human chemistry, Romantic chemistry, (d) Relationship chemistry, (e) Social chemistry, (f) Interpersonal chemistry, (g) Sexual chemistry, (h) Physical chemistry, (i) Neurochemistry, (j) Queer chemistry,
2. LoveIsSexy (About) – LoveIsSexy.com.
3. (a) Thims, Libb. (2007). Human Chemistry (Volume One), (preview), (Google books). Morrisville, NC: LuLu.
(a) Thims, Libb. (2007). Human Chemistry (Volume Two), (ch. 16: Human Thermodynamics, pgs. 653-702), (preview), (Google books). Morrisville, NC: LuLu.
4. Orr, Andrea. (2003). Meeting, Mating, and Cheating: Sex, Love, and the New World of Online Dating, (ch. 4: Love Minus Chemistry Equals Friendship, pgs. 53-62). Reuters Prentice Hall.
5. Goethe, Johann. (1809). Elective Affinities. New York: Penguin Classics.