Do you have a cubic cell? Don't do VariableCell then, it destroys symmetry. (check the forces on atoms to be 0)If you know the exact symmetry of the cell (relative positions of atoms in the cell) then adjust only lattice constant.
Then the effect of pressure in all direction will change the lattice equivalently as well. With the same relative coordinates just change the lattice constant in the range +-3% with the step 0.25-0.5% and check the total energies.
