mirror of https://gitlab.com/QEF/q-e.git
117 lines
3.4 KiB
Fortran
117 lines
3.4 KiB
Fortran
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!
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! Copyright (C) 2001 PWSCF group
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! This file is distributed under the terms of the
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! GNU General Public License. See the file `License'
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! in the root directory of the present distribution,
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! or http://www.gnu.org/copyleft/gpl.txt .
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!
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!
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!--------------------------------------------------------------------
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subroutine efermit (et, nbndx, nbnd, nks, nelec, nspin, ntetra, &
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tetra, ef)
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!--------------------------------------------------------------------
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!
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! Finds the Fermi energy - tetrahedron method (Bloechl)
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!
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use parameters
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implicit none
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integer :: nks, nbndx, nbnd, nspin, ntetra, tetra (4, ntetra)
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! input: the number of k points
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! input: the maximum number of bands
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! input: the number of bands
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! input: the number of spin components
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! input: the number of tetrahedra
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! input: the vertices of a tetrahedron
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real(kind=DP) :: et (nbndx, nks), nelec, ef
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! input: the eigenvalues
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! input: the number of electrons
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! output: the fermi energy
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!
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! two parameters
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!
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integer :: maxiter
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! the maximum number of iterations in
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real(kind=DP) :: rydtoev, eps
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! the transformation Ry to eV
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! a small quantity
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parameter (maxiter = 300, rydtoev = 13.6058d0, eps = 1.0d-10)
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!
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! here the local variables
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!
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integer :: nlw, ik, iter
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! the minimum energy band
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! counter on k points
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! counter on iterations
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real(kind=DP) :: elw, eup, sumkup, sumklw, sumkt, sumkmid
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! the lower limit of the fermi ener
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! the upper limit of the fermi ener
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! the number of states with eup
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! the number of states with elw
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! the sum over the states below E_f
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! the number of states with ef
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real(kind=DP) :: efbetter, better
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external sumkt, error
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!
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! find bounds for the Fermi energy.
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!
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nlw = max (1, nint (nelec / 2.0 - 5.0) )
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elw = et (nlw, 1)
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eup = et (nbnd, 1)
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do ik = 2, nks
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elw = min (elw, et (nlw, ik) )
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eup = max (eup, et (nbnd, ik) )
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enddo
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!
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! Bisection method
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!
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sumkup = sumkt (et, nbndx, nbnd, nks, nspin, ntetra, tetra, eup)
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sumklw = sumkt (et, nbndx, nbnd, nks, nspin, ntetra, tetra, elw)
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if ( (sumkup - nelec) .lt. - eps.or. (sumklw - nelec) .gt.eps) &
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call error ('efermit', 'unexpected error', 1)
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better = 1.0e+10
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do iter = 1, maxiter
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ef = (eup + elw) / 2.0
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sumkmid = sumkt (et, nbndx, nbnd, nks, nspin, ntetra, tetra, ef)
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if (abs (sumkmid-nelec) .lt.better) then
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better = abs (sumkmid-nelec)
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efbetter = ef
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endif
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! converged
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if (abs (sumkmid-nelec) .lt.eps) then
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goto 100
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elseif ( (sumkmid-nelec) .lt. - eps) then
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elw = ef
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else
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eup = ef
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endif
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enddo
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! unconverged exit:
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! the best available ef is used . Needed in some difficult cases
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ef = efbetter
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sumkmid = sumkt (et, nbndx, nbnd, nks, nspin, ntetra, tetra, ef)
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write (6, 9010) ef * rydtoev, sumkmid
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! converged exit:
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100 continue
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! Check if Fermi level is above any of the highest eigenvalues
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do ik = 1, nks
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if (ef.gt.et (nbnd, ik) + 1.d-4) write (6, 9020) ef * rydtoev, ik, &
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et (nbnd, ik) * rydtoev
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enddo
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return
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9010 format (/5x,'Warning: too many iterations in bisection'/ &
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& 5x,'ef = ',f10.6,' sumk = ',f10.6,' electrons')
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9020 format (/5x,'Warning: ef =',f10.6, &
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& ' is above the highest band at k-point',i4,/5x,9x, &
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& 'e = ',f10.6)
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end subroutine efermit
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