Since MgO has a cubic cell and all the atoms are in special positions, the only degree of freedom is the lattice parameter. Then, a simple geometry optimisation can be devised.
Draw the total energy versus the lattice parameter (x-y chart) and locate the equilibrium lattice parameter.
Hints: Span the lattice parameter in the region: 4.12 - 4.26 (15 pts). Use the xmgr (or gnuplot) program to plot the curve.