  An ultracold Fermi gas with a zero-range attractive potential in the unitary
limit is investigated using variational and diffusion Monte Carlo methods.
Previous calculations have used a finite range interactions and extrapolate the
results to zero-range. Here we extend the quantum Monte Carlo method to
directly use a zero-range interaction without extrapolation. We employ a trial
wave function with the correct boundary conditions, and modify the sampling
procedures to handle the zero-range interaction. The results are reliable and
have low variance.
