I understand that the lattice energy is the energy released when an solid ionic compound forms or it is the energy required to separate completely a mole of a solid ionic compound into its gaseous ions. So is second definition the same thing as vaporizing an ionic compound and so we can directly use the lattice energy instead of the calculations we usually do ? For example, I tried to calculate the energy required to vaporize one mole of Magnesium oxide (MgO) using the ordinary method. I got the melting and boiling points, latent heat of fusion and vaporization from here: http://www.microkat.gr/msdspd90-99/Magnesium oxide.htm. I then calulated the energy to raise the temperature to melting point + the heat of fusion + the energy to raise the temperature to boiling point + and the heat of vaporization. The result was 560 kJ/mole of MgO. By trying to do the same using the lattice energy approach, I found that the lattice energy of MgO is 3800 kJ/mole, which is about 7 times the calculated energy above. So my questions here are: 1- Is the second definition of lattice energy the same as the energy needed to vaporize an ionic compound and so they can be used interchangeably ? 2- If the answer to question (1) is yes, then why don't the two approaches give the same result ?