  We investigate the time evolution of the gravitational potential Phi for a
special class of non-adiabatic Unified Dark matter Models described by scalar
field lagrangians. These models predict the same background evolution as in the
LambdaCDM and possess a non-vanishing speed of sound. We provide a very
accurate approximation of Phi, valid after the recombination epoch, in the form
of a Bessel function of the first kind. This approximation may be useful for a
future deeper analysis of Unified Dark Matter scalar field models.
