Evolution of the number density of molecular hydrogen H₂ for different initial metallicity Z and options for the cooling by metals. The meaning of the lines is as follows: high Z and no cooling by metals (solid line), low Z and no cooling by metals (dotted line), high Z and cooling by metals enabled (dashed line), low Z and cooling by metals switched off (dashed-dotted line). The dashed-dotted and dotted lines overlap. The cooling by Cen (1992) is switched off in both panels.