  We introduce a new tool - AESOPUS: Accurate Equation of State and OPacity
Utility Software - for computing the equation of state and the Rosseland mean
(RM) opacities of matter in the ideal gas phase. Results are given as a
function of one pair of state variables, (i.e. temperature T in the range 3.2
<= log(T) <= 4.5, and parameter R= rho/(T/10^6 K)^3 in the range -8 <= log(R)
<= 1), and arbitrary chemical mixture. The chemistry is presently solved for
about 800 species, consisting of almost 300 atomic and 500 molecular species.
The gas opacities account for many continuum and discrete sources, including
atomic opacities, molecular absorption bands, and collision-induced absorption.
Several tests made on AESOPUS have proved that the new opacity tool is accurate
in the results,flexible in the management of the input prescriptions, and agile
in terms of computational time requirement. We set up a web-interface
(http://stev.oapd.inaf.it/aesopus) which enables the user to compute and
shortly retrieve RM opacity tables according to his/her specific needs,
allowing a full degree of freedom in specifying the chemical composition of the
gas. Useful applications may regard RM opacities of gas mixtures with i)
scaled-solar abundances of metals, choosing among various solar mixture
compilations available in the literature; ii) varying CNO abundances, suitable
for evolutionary models of red and asymptotic giant branch stars and massive
stars in the Wolf-Rayet stages; iii) various degrees of enhancement in
alpha-elements, and C-N, Na-O and Mg-Al abundance anti-correlations, necessary
to properly describe the properties of stars in early-type galaxies and
Galactic globular clusters; iv) zero-metal abundances appropriate for studies
of gas opacity in primordial conditions.
