QE代码的bug（费米面下拟合能带电子数nelec错误）

QE代码的bug（费米面下拟合能带电子数错误）

EPW计算的Ef有问题：只有在wannier拟合时，project的能带加exclude_bands能带包含了费米能以下的所有能带，在插值时才能得到正确的费米能级。可以选择将费米面下不需要拟合的能带全部排除出去，或者将不能排除出去的能带全部拟合。该问题是由于原始的在ephwann_shuffle.f90和ephwann_shuffle_mem.f90代码中对于nelec不能正确计算导致。

  WRITE(stdout,'(/5x,a,f10.6,a)') 'Fermi energy coarse grid = ', ef * ryd2ev, ' eV'
  !
  IF (efermi_read) THEN
    !
    ef = fermi_energy
    WRITE(stdout,'(/5x,a)') REPEAT('=',67)
    WRITE(stdout, '(/5x,a,f10.6,a)') 'Fermi energy is read from the input file: Ef = ', ef * ryd2ev, ' eV'
    WRITE(stdout,'(/5x,a)') REPEAT('=',67)
    !
    ! SP: even when reading from input the number of electron needs to be correct
    already_skipped = .FALSE.
    IF (nbndskip > 0) THEN
      IF (.NOT. already_skipped) THEN
        IF (noncolin) THEN
          nelec = nelec - one * nbndskip
        ELSE
          nelec = nelec - two * nbndskip
        ENDIF
        already_skipped = .TRUE.
        WRITE(stdout, '(/5x,"Skipping the first ", i4, " bands:")') nbndskip
        WRITE(stdout, '(/5x,"The Fermi level will be determined with ", f9.5, " electrons")') nelec
      ENDIF
    ENDIF
    !
  ELSEIF (band_plot) THEN
    !
    WRITE(stdout, '(/5x,a)') REPEAT('=',67)
    WRITE(stdout, '(/5x,"Fermi energy corresponds to the coarse k-mesh")')
    WRITE(stdout, '(/5x,a)') REPEAT('=',67)
    !
  ELSE
    ! here we take into account that we may skip bands when we wannierize
    ! (spin-unpolarized)
    ! RM - add the noncolin case
    already_skipped = .FALSE.
    IF (nbndskip > 0) THEN
      IF (.NOT. already_skipped) THEN
        IF (noncolin) THEN
          nelec = nelec - one * nbndskip
        ELSE
          nelec = nelec - two * nbndskip
        ENDIF
        already_skipped = .TRUE.
        WRITE(stdout, '(/5x,"Skipping the first ", i4, " bands:")') nbndskip
        WRITE(stdout, '(/5x,"The Fermi level will be determined with ", f9.5, " electrons")') nelec
      ENDIF
    ENDIF
    !
    ! Fermi energy
    !
    ! Since wkf(:,ikq) = 0 these bands do not bring any contribution to Fermi level
    IF (ABS(degaussw) < eps16) THEN
      ! Use 1 meV instead
      efnew = efermig(etf, nbndsub, nkqf, nelec, wkf, 1.0d0 / ryd2mev, ngaussw, 0, isk_dummy)
    ELSE
      efnew = efermig(etf, nbndsub, nkqf, nelec, wkf, degaussw, ngaussw, 0, isk_dummy)
    ENDIF
    !
    WRITE(stdout, '(/5x,a,f10.6,a)') &
        'Fermi energy is calculated from the fine k-mesh: Ef = ', efnew * ryd2ev, ' eV'
    !
    ! if 'fine' Fermi level differs by more than 250 meV, there is probably something wrong
    ! with the wannier functions, or 'coarse' Fermi level is inaccurate
    IF (ABS(efnew - ef) * ryd2eV > 0.250d0 .AND. (.NOT. eig_read)) &
      WRITE(stdout,'(/5x,a)') 'Warning: check if difference with Fermi level fine grid makes sense'
    WRITE(stdout,'(/5x,a)') REPEAT('=',67)
    !
    ef = efnew
  ENDIF

其中nelec需要使用wannier拟合的能带中包含的电子占据数代替。该问题容易导致计算得到的插值空间中的Ef是错误的，同时可能会导致如下报错：

Warning: check if difference with Fermi level fine grid makes sense

我们通过引入新的参数nbndnfbfe进行修正。

1. 在epwcom.f90中添加变量nbndnfbfe

integer :: nbndnfbfe
!! number of band not wannier fitting below fermi energy

nelec-1

1. 在epw_readin.f90中添加对变量nbndnfbfe的设置
- 2.1 在USE epwcom, only : 中添加nbndnfbfe
- 2.2 在NAMELIST / inputepw /中添加nbndnfbfe
- 2.3 在607行左右设置nbndnfbfe的初始值为0
- 2.4 在1000行左右，将nbndnfbfe的值传递给所有核。 call mp_bcast(nbndnfbfe,meta_ionode_id,world_comm)

在ephwann_shuffle.f90和ephwann_shuffle_mem.f90中添加nbndnfbfe,并根据nbndnfbfe对nelec进行修正。

3.1 在ephwann_shuffle.f90和ephwann_shuffle_mem.f90的USE epwcom, only : 中添加nbndnfbfe
3.2 在ephwann_shuffle.f90和ephwann_shuffle_mem.f90的下列语句之间添加对nelec的修正：

WRITE(stdout,'(/5x,a,f17.10,a)') 'Fermi energy coarse grid = ', ef * ryd2ev, ' eV'	
!
IF (efermi_read) THEN

改为：

WRITE(stdout,'(/5x,a,f17.10,a)') 'Fermi energy coarse grid = ', ef * ryd2ev, ' eV'
!
IF (noncolin) THEN
  nelec = nelec - one * nbndnfbfe
ELSE
  nelec = nelec - two * nbndnfbfe
ENDIF
!
IF (efermi_read) THEN

1. 则在epw的输入文件中可以使用参数nbndnfbfe来指定费米面以下未被Wannier拟合且未被exclude_bands的能带条数来避免这个bug。输入文件如下：

wannierize = .true.
nbndsub = 2
nbndnfbfe = 3
bands_skipped = 'exclude_bands = 1:2'

1. 修改后代码以及软件编译参考：QUANTUM ESPRESSO v7.1 修改版安装

QE代码的bug（费米面下拟合能带电子数错误）

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