  We study the ground state of a trapped Bose gas, starting from the full
many-body Schr{\"o}dinger Hamiltonian, and derive the nonlinear Schr{\"o}dinger
energy functional in the limit of large particle number, when the interaction
potential converges slowly to a Dirac delta function. Our method is based on
quantitative estimates on the discrepancy between the full many-body energy and
its mean-field approximation using Hartree states. These are proved using
finite dimensional localization and a quantitative version of the quantum de
Finetti theorem. Our approach covers the case of attractive interactions in the
regime of stability. In particular, our main new result is a derivation of the
2D attractive nonlinear Schr{\"o}dinger ground state.
