fermi -Fermi occupation control variable
The fermi variable determines the use of a Fermi distribution
for the calculation of occupation numbers of the Kohn-Sham orbitals when
empty states are included in the calculation (nempty
> 0). When fermi is set to ON, the occupation numbers are
calculated from the Kohn-Sham eigenvalues e(i) using the expression
n(i) = 1 / ( 1 + exp( ( e(i) - mu )/ (k * fermi_temp) )where the chemical potential mu is adjusted so as to conserve total electronic charge.
When fermi is set to OFF, (the default), occupation numbers are calculated by filling states with 2 electrons (1 if spin is ON) until all electrons are used.