Chemistry. It's a word tossed around in plenty of movie reviews, generally to diagnose whether two actors have it or not. Chemistry can be in the eye of the beholder: some critics may disagree, but if the chemistry is really there, it will show in the way the film catches on. No one can deny that Billy Crystal and Meg Ryan had it in When Harry Met Sally, or that William Powell and Myrna Loy, or Woody Allen and Diane Keaton had it in their many films. But for every hit, there are many, many failed experiments. Here are seven of the most (or least) memorable.

1. Fred Astaire and Joan Fontaine in A Damsel in Distress (1937)
For whatever reason, Astaire decided to break up his hit formula with Ginger Rogers and make this movie without her. His new partner? Joan Fontaine, best known for playing mousy, quietly pretty types (Rebecca, Letter from an Unknown Woman, etc.) and definitely not a song-and-dance woman. Poor Joan was taken to the mat for her lack of singing and hoofing, although the film actually isn't that bad. The ultra-witty P.G. Wodehouse adapted his own novel, it won an Oscar for its dance choreography, and it features another great team: George Burns and Gracie Allen.

2. Hugh Grant and Sandra Bullock in Two Weeks Notice (2002)
Every so often some Hollywood executive gets the idea to team up two big stars, hoping that their massive appeal will translate into screen chemistry; it mostly doesn't. These two romantic comedy masters, who have been wonderful in other films with other people, came together like a dull, wet flint, unable to strike even the most meager spark. Another infamous example of this type of casting came in 1986: recent Oscar winners Jack Nicholson and Meryl Streep came together for Mike Nichols' Heartburn. Before it opened, it had lots of buzz. After it opened, it had more of a stench.



3. Audrey Hepburn & Humphrey Bogart in Sabrina (1954)
This is one of Hepburn's strangest films. She's at her absolute best, perfectly at home with Billy Wilder's wit, but playing in a love triangle with two men with whom she hasn't the least connection. The prickly William Holden is supposed to be the slick ladies' man, and then she's supposed to end up with the more responsible Bogart (whom she "loosens up"). But Bogey was past fifty, slowing down and near the end of his life (he died of cancer three years later), and Audrey was in her sparkly twenties. Four decades later, Sydney Pollack didn't fare much better with his lethargic remake, starring Julia Ormond, Harrison Ford and Greg Kinnear.

4. Matthew McConaughey and Kate Hudson in Fool's Gold (2008)
I haven't seen this movie; I very cleverly talked my way into attending the Welcome Home Roscoe Jenkins press screening on the same night (I think I got the better end of the deal). But I have to say that the American cinema has rarely produced two more vacuous "stars" than McConaughey and Hudson. The only good thing about this film is that by appearing together, they only wasted the resources of one movie rather than churning out two separate pieces of garbage. They can have each other.

5. Scarlett Johansson and Chris Evans in The Nanny Diaries (2007)
Nine times out of ten when a romantic matchup doesn't work, it's because of the guy. Either he's too self-centered (Ben Stiller), too old, or too much of a schlub (Adam Sandler and Kate Beckinsale in Click), or just completely without a screen personality. Last summer I saw four Chris Evans movies in the space of two months, and I could never remember who he was or where I'd seen him before. You have to ask: how hard would it be to strike up some kind of erotic energy with the amazing Scarlett? But no: Evans just fades away into the drapery. I suppose he's someone's idea of cute -- he plays "The Harvard Hottie" in this film -- but the camera doesn't care for cute unless there's something behind it.

6. Ben Affleck and Jennifer Lopez in Gigli (2003)
Okay, so maybe we overreacted a bit when the critical community mercilessly panned this movie (I gave it 0 stars out of 4). It's an awful movie all right, but it's so bizarre and clueless that it at least has some points of interest, first and foremost being Christopher Walken's astonishing monologue about Marie Calendar. My point, however, is that real-life couples tend not to generate any chemistry in movies. They're already comfortable with one another, so the charge is gone. It's better when a couple falls in love during filming, as with Humphrey Bogart and Lauren Bacall on To Have and Have Not, or when the director uses the real-life marriage to a dramatic advantage, as in Eyes Wide Shut. But Affleck and Lopez have so little going on here that you wonder what they could have been thinking both onscreen and off.

7. Sylvester Stallone and Dolly Parton in Rhinestone (1984)
If we could tear any one section away from movie history and dispose of it forever, it might be Sylvester Stallone's attempts at comedy (Stop! Or My Mom Will Shoot, Spy Kids 3, etc.). That goes double for his attempts at screen romance, starting with this version of Pygmalion in which Dolly Parton tries to transform a vulgar cabbie (Stallone) into a country singer. At best he and Parton could both be considered cartoon characters, with enlarged body parts, rather than trying to create any genuine chemistry. At worst, it's just painful.