# [DFTB-Plus-User] How to specify electronic energy in the input?

Lipp, Vladimir vladimir.lipp at desy.de
Wed Aug 28 16:51:01 CEST 2019

```Dear Ben,

Thank you for your reply!

Yes, I need to specify the kinetic energy of the electrons. My simulation starts with Te=300 K and I am slowly adding energy to the electrons. I know how much energy I will add, but I don't know which temperature will be reached. The corresponding temperature will of course oscillate with the atomic motion; this is expected for the NVE ensemble. But the energy should not oscillate as there is no thermostat or exchange of the energy with anything.

Currently, the electron energy is oscillating instead, which prevents me from achieving the energy conservation. The electron temperature may reach a very high value in my case (let's say 1.5 eV in diamond or more), I clearly see the problem coming from the TS term. Is this possible to calculate the forces from U, not U-TS?

Vladimir

----- Original Message -----
From: "Benjamin Hourahine" <benjamin.hourahine at strath.ac.uk>
To: "dftb-plus-user" <dftb-plus-user at mailman.zfn.uni-bremen.de>
Sent: Wednesday, 28 August, 2019 15:59:46
Subject: Re: [DFTB-Plus-User] How to specify electronic energy in the input?

Hello Vladimir,

What do you mean by specifying the energy of the electrons? The internal
electronic energy (and hence the Mermin free energy) is a functional of
the coordinates of the atoms, so will change on moving atoms about. Or
are you asking if there is a way to set the kinetic energy of the
electrons above the zero temperature ground state?

The internal energy and also the extrapolated zero temperature energy is
available in the output (depending on how you are calling DFTB+, this
might already be directly available to you now).  The reason for
returning the Mermin energy (at the specified temperature) is that the
Helman-Feynmann forces are the derivative of this energy and not the
other cases.

Unless your system is either at very high temperature or has a very
unusual electronic structure, the Mermin TS term is very small.

Regards

Ben

without the electron entropy correction

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