Author Topic: freeEnergy()  (Read 73 times)

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Offline k.cerda

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freeEnergy()
« on: November 1, 2021, 23:48 »
Hi

The potential energy calculated by TotalEnergy in ATK is always negative. But freeEnergy() calculated from phonons is positive. Why is this difference? Thanks.

Offline sureshko

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Re: freeEnergy()
« Reply #1 on: November 2, 2021, 16:15 »
Hi Cerda,
Historically, the attraction between atoms is measured as a negative value in energy units and repulsion/destabilization as a positive value (remember Lennard-Jones potential form). Therefore, any stable system of interacting atoms will have negative potential energy due to the net attraction/bonding. The freeEnergy() function from PhononDOS gives U - TS (without the potential energy) and this can be positive or negative depending on the temperature. At 0 K, there is no entropy and U=zero point energy, so pdos.freeEnergy(0*Kelvin) will be positive because ZPE is positive. As T increases, the entropic contribution to free energy increases, and free energy becomes negative when -TS overcomes U. I hope the above answers your question.
With regards,
Suresh

Offline k.cerda

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Re: freeEnergy()
« Reply #2 on: November 2, 2021, 23:42 »
Dear shuresko

Thanks a lot for the helpful answer. Best regards.