[NP completeness in] natural systems
Finding minimum energy configurations is formally NP-complete in standard models of natural systems such as folding protein and DNA molecules (see page 1003), collections of charges on a sphere (compare page 987), and finite regions of spin glasses (see page 944). As discussed on page 351, however, it seems likely that in nature true minima are very rare, and that instead what is usually seen are just the results of actual dynamical processes of evolution.
In quantum field theory and to a lesser extent quantum mechanics and celestial mechanics, approximation schemes based on perturbation series seem to require computations that grow very rapidly with order. But exactly what this implies about the underlying physical processes is not clear.