Non-integer total number of Alpha/Beta Electrons
M. Brehm
brehmin... at googlemail.com
Tue Feb 17 19:50:53 UTC 2015
Dear Community,
I am performing DFT calculations of a solid-state lattice with CP2k, using
the classical mixing scheme and Fermi-Dirac-Smearing, like usual for
lattices. As the total number of electrons in my system is odd, I perform
unrestricted DFT calculations, setting the spin multiplicity to 2. In the
beginning, CP2k displays 288 alpha electrons and 287 beta electrons, like
it should be. The SCF cycle converges, and in the end, I print out the MO
energies and occupation numbers. Suddenly, there are 287.5 alpha electrons
and 287.5 beta electrons! For my understanding, the total number of
alpha/beta electrons is not a degree of freedom of a unrestricted DFT
calculation, and should remain fixed during the calculations. How can it be
that in the end I get these fractional electron numbers? With slightly
different systems or geometries, this is not the case, and the total
electron count for alpha/beta remains constant.
I assumed that it could have something to do with the initial SCF guess
(poor guess might lead to convergence into a local minimum which is not the
ground state), but I re-ran the calculation with SCF guess "random", and
still get exactly the same results.
My input file and output file are attached to this posting.
Any hints/comments are appreciated.
Best regards,
Martin
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