QuantumATK Forum

QuantumATK => General Questions and Answers => Topic started by: k.cerda on November 1, 2021, 23:48

Title: freeEnergy()
Post by: k.cerda 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.

Title: Re: freeEnergy()
Post by: sureshko 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

Title: Re: freeEnergy()
Post by: k.cerda on November 2, 2021, 23:42

Dear shuresko

Thanks a lot for the helpful answer. Best regards.