The trimer reconstruction of the (100) InP surface which has been discovere
d experimentally is confirmed by first-principle calculations. The charge d
ensity of atomic configuration, which has the lowest surface energy is in p
erfect agreement with experimental scanning tunneling microscopy images. We
predict the same trimer reconstruction also to be observable on GaP (100)
surfaces and discuss how local stress makes this reconstruction energetical
ly unfavorable for GaAs. [S0163-1829(99)01943-8].