Models are presented for the polytropic equation of state of self-gravitating , quiescent interstellar gas clouds .
A detailed analysis , including chemistry , thermal balance , and radiative transfer , is performed for the physical state of the gas as a function of density , metallicity , velocity field , and background radiation field .
It is found that the stiffness of the equation of state strongly depends on all these physical parameters , and the adiabatic index varies between \sim 0.2 - 1.4 .
The implications for star formation , in particular at high redshift and in starburst galaxies , and the initial stellar mass function are discussed .