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**Questions and Answers / Re: Presure calculations**

« **on:**April 14, 2015, 09:36 »

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.

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.