I contributed; Ulam did not. I'm sorry I had to answer it in this abrupt way. Ulam was rightly dissatisfied with an old approach. He came to me with a part of an idea which I already had worked out and difficulty getting people to listen to. He was willing to sign a paper. When it then came to defending that paper and really putting work into it, he refused. He said, "I don't believe in it."
On the creation of the hydrogen bomb, in "Infamy and honor at the Atomic Café : Edward Teller has no regrets about his contentious career" by Gary Stix in Scientific American (October 1999), p. 42-43.