Natural Quintessence?
  We formulate conditions for the naturalness of cosmological quintessence scenarios. The quintessence lagrangian is taken to be the sum of a simple exponential potential and a non-canonical kinetic term. This parameterization covers most variants of quintessence and makes the naturalness conditions particularly transparent. Several ``natural'' scalar models lead, for the present cosmological era, to a large fraction of homogeneous dark energy density and an acceleration of the scale factor as suggested by observation. 
A. Hebecker, C. Wetterich