How is the "force on cell" calculated

[Tamidor]

Below is an extract of the OUTCAR file for an MgO calculation


FORCE on cell =-STRESS in cart. coord. units (eV/reduce length):
Direction X Y Z XY YZ ZX
-------------------------------------------------------------------------------------
Alpha Z 199.46 199.46 199.46
Ewald -1294.83 -1294.83 -1294.83 0.00 0.00 0.00
Hartree 424.77 424.77 424.77 0.00 0.00 0.00
E(xc) -310.61 -310.61 -310.61 0.00 0.00 0.00
Local 154.15 154.15 154.15 0.00 0.00 0.00
n-local -121.59 -122.33 -121.58 0.07 0.02 0.02
augment 258.54 258.54 258.54 0.00 0.00 0.00
Kinetic 690.01 695.35 698.54 -3.85 -1.57 -1.57
-------------------------------------------------------------------------------------
Total 4.29 4.29 4.29 0.00 0.00 0.00
in kB 94.45 94.45 94.45 0.00 0.00 0.00
external pressure = 94.45 kB Pullay stress = 0.00 kB

I don´t really understand how the force on the cell is calculated. When I look at the different energy contributions they are not always equal in x y z but still the Total is the same for every direction, which it should be for MgO. Also I don´t now how the value of 4.29 is calculated from the different contributions.

When I do several calculations for different lattice constants and calulate the Forces by the derivative of an energy vs lattice constant plot I get different values.

Thank you in advance
Tamidor