[Pw_forum] A little question about total energy
marzari at MIT.EDU
Sat May 17 19:25:49 CEST 2003
To add to this - when you have a broadening the PWSCF
total energy is what others call the total free energy F=E-TS.
E.g., if you choose a Fermi-Dirac broadening, the PWSCF
total energy is really the Mermin-Kohn-Sham free energy.
Last, if you are studying isolated atoms, but still use a
broadening, you have an entropy S different from zero if there
are degeneracies giving rise to fractional occupation numbers.
So, you might want to remove the -TS (the correction for metals),
and also make sure your results are converged in the limit of
small T (i.e. small broadening temperatures).
Paolo Giannozzi wrote:
>> In the output of PWSCF, the energy partitions are list
>>following the total energy as I pasted below. I found
>>that the total energy is not equal to the sum of those listed
> Total Energy = sum of terms marked with "+" below =
> one-electron + hartree + xc + ewald + correction for metals
> ! total energy = -62.07569469 ryd
> estimated scf accuracy < 0.00000799 ryd
> band energy sum = -14.82580512 ryd
> + one-electron contribution = -98.79712762 ryd
> + hartree contribution = 51.10053218 ryd
> + xc contribution = -14.02297161 ryd
> + ewald contribution = -0.34484385 ryd
> scf in/out correction = 0.02901784 ryd
> + correction for metals = -0.01128379 ryd
> The "one-electron contribution" is the kinetic + ionic term:
> \sum <psi| T + V_ion | psi>
> The "band energy sum" is exactly what it claims to be:
> \sum epsilon = \sum <psi| T + V_ion + V_scf | psi>
> The "correction for metals" makes forces and energies
> consistent if a broadening scheme is used.
> The "scf in/out correction" is no longer used
> (and no longer useful)
> The "estimated scf accuracy" gives an estimate of the
> error on the energy due to imperfect self-consistency.
> Admittedly, it is slightly confusing.
Prof Nicola Marzari Department of Materials Science and Engineering
13-5066 MIT 77 Massachusetts Avenue Cambridge MA 02139-4307 USA
tel 617.4522758 fax 617.2586534 marzari at mit.edu http://nnn.mit.edu
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