  We have studied the structural and electronic properties of Mg(1-x)Al(x)B2
within the Virtual Crystal Approximation (VCA) by means of first-principles
total-energy calculations. Results for the lattice parameters, the electronic
band structure, and the Fermi surface as a function of Al doping for 0<x<0.6
are presented. The ab initio VCA calculations are in excellent agreement with
the experimentally observed change in the lattice parameters of Al doped MgB2.
The calculations show that the Fermi surface associated with holes a the boron
planes collapses gradually with aluminum doping and vanishes for x=0.56. In
addition, an abrupt topological change in the sigma-band Fermi surface was
found for x=0.3. The calculated hole density correlates closely with existing
experimental data for Tc(x), indicating that the observed loss of
superconductivity in Mg(1-x)Al(x)B2 is a result of hole bands filling.
