  A generalized diffusion Monte Carlo method for solving the many-body
Schr\"odinger equation on curved manifolds is introduced and used to perform a
`fixed-phase' simulation of the fractional quantum Hall effect on the Haldane
sphere. This new method is used to study the effect of Landau level mixing on
the $\nu=1/3$ energy gap and the relative stability of spin-polarized and
spin-reversed quasielectron excitations.
