  Determining the equation of state of dark energy with astronomical
observations is crucially important to understand the nature of dark energy. In
performing a likelihood analysis of the data, especially of the cosmic
microwave background and large scale structure data the dark energy
perturbations have to be taken into account both for theoretical consistency
and for numerical accuracy. Usually, one assumes in the global fitting analysis
that the dark energy perturbations are adiabatic. In this paper, we study the
dark energy isocurvature perturbation analytically and discuss its implications
for the cosmic microwave background radiation and large scale structure.
Furthermore, with the current astronomical observational data and by employing
Markov Chain Monte Carlo method, we perform a global analysis of cosmological
parameters assuming general initial conditions for the dark energy
perturbations. The results show that the dark energy isocurvature perturbations
are very weakly constrained and that purely adiabatic initial conditions are
consistent with the data.
