Most molecular modeling software packages allow the user to choose a value for the electron density, often called the IsoValue. Some software[3] also allows for specification of the electron density in terms of percentage of total electrons enclosed. Depending on the IsoValue (typical units are electrons per cubic bohr), or the percentage of total electrons enclosed, the electron density surface can be used to locate atoms, emphasize electron densities associated with chemical bonds, or to indicate overall molecular size and shape.[4]