  We determine the regime where the widespread classical field description for
quantum Bose gases is quantitatively accurate in 1d, 2d, and 3d by a careful
study of the ideal gas limit. Numerical benchmarking in 1d shows that the ideal
gas results carry over unchanged into the weakly interacting gas. The optimum
high energy cutoff is in general shown to depend strongly on the observable in
question (e.g. energy, density fluctuations, phase coherence length, condensate
fraction). This explains the wide spread of past results. A consistent
classical field representation with less than 10% deviation in all typical
observables can be given for systems at temperatures below 0.0064 degeneracy
temperature in 1d, and 0.49 critical temperature in 3d. Surprisingly, this is
not possible for the 2d ideal gas even at zero temperature because mean
density, density fluctuations and energy cannot be simultaneously matched to
the quantum results.
