For a static system (i.e. obtained from a geometry optimization), the pressure can be calculated from the stress tensor via P = -1/3 (s_xx + s_yy + s_zz) (see attached script).
At finite temperature (i.e. in a molecular dynamics run) there would be an additional contribution of the kinetic energy: P = 2/3 E_kin/V - 1/3 (s_xx + s_yy + s_zz).
If you want to compare the enthalpy of two systems, you have to simulate them at the desired target pressure. In a molecular dynamics simulation, this can easily be achieved by employing one the NPT-methods. If you intend to obtain the structures via a geometry optimization, you have to optimize the cell until the desired target stress is reached. In ATK2015, this can easily be achieved via the target_stress parameter.
