From the conversion of units:
1 cm^3 is equivalent to 1 mL
1 L is equivalent to 1000 mL
therefore,
to convert from liter to cm^3, we simply multiply by 1000.
Note that the multiplication will be done in the denominator.
Based on this:
density = (0.625 g/l) x (1g/1000 cm^3) = 0.000625 g/cm^3
= 6.25 x 10^-4 g/cm^3