Line data    Source code

       1              : !!****m* ABINIT/m_vcoul/m_cutoff_sphere
       2              : !! NAME
       3              : !!  m_cutoff_sphere
       4              : !!
       5              : !! FUNCTION
       6              : !!
       7              : !! SOURCE
       8              : 
       9              : #if defined HAVE_CONFIG_H
      10              : #include "config.h"
      11              : #endif
      12              : 
      13              : 
      14              : module m_cutoff_sphere
      15              : 
      16              :  use defs_basis
      17              :  use m_abicore
      18              :  use m_geometry,        only : normv
      19              : 
      20              :  implicit none
      21              : 
      22              :  private
      23              : 
      24              :  public ::  cutoff_sphere
      25              : !!***
      26              : 
      27              : CONTAINS  !========================================================================================
      28              : !!***
      29              : 
      30              : !!****f* m_cutoff/cutoff_sphere
      31              : !! NAME
      32              : !! cutoff_sphere
      33              : !!
      34              : !! FUNCTION
      35              : !!  Calculate the Fourier transform of the Coulomb interaction with a spherical cutoff in real-space.
      36              : !!
      37              : !!   $ v_{cut}(G)= \frac{4\pi}{|q+G|^2} [ 1-cos(|q+G|*R_cut) ] $  (1)
      38              : !!
      39              : !!  For |q|<small and G=0 we use 2pi.R_cut^2, namely we consider the limit q-->0 of Eq. (1)
      40              : !!
      41              : !! INPUTS
      42              : !!  qpt(3)=q-point where the cutoff Coulomb is required.
      43              : !!  ngvec=Number of G vectors
      44              : !!  gvec(3,ngvec)=G vectors in reduced coordinates.
      45              : !!  gmet(3,3)=Metric in reciprocal space.
      46              : !!  rcut=Cutoff radius of the sphere.
      47              : !!
      48              : !! OUTPUT
      49              : !!  vc_cut(ngvec)=Fourier components of the effective Coulomb interaction.
      50              : !!
      51              : !! SOURCE
      52              : 
      53          137 : subroutine cutoff_sphere(qpt, ngvec, gvec, gmet, rcut, vc_cut)
      54              : 
      55              : !Arguments ------------------------------------
      56              : !scalars
      57              :  integer,intent(in) :: ngvec
      58              :  real(dp),intent(in) :: rcut
      59              : !arrays
      60              :  integer,intent(in) :: gvec(3,ngvec)
      61              :  real(dp),intent(in) :: gmet(3,3),qpt(3)
      62              :  real(dp),intent(out) :: vc_cut(ngvec)
      63              : 
      64              : !Local variables-------------------------------
      65              : !scalars
      66              :  integer :: ig,igs
      67              :  real(dp) :: qpg
      68              :  logical :: ltest
      69              : 
      70              : !************************************************************************
      71              : 
      72              :  !------------------------------------------------------------
      73              :  ! Code modification by Bruno Rousseau, Montreal, 06/11/2013
      74              :  !------------------------------------------------------------
      75              :  !
      76              :  ! In order for the code below to run in parallel using MPI
      77              :  ! within the gwls code (by Laflamme-Jansen, Cote and Rousseau)
      78              :  ! the ABI_CHECK below must be removed.
      79              :  !
      80              :  ! The ABI_CHECK below will fail if G-vectors are distributed on
      81              :  ! many processors, as only the master process has G=(0,0,0).
      82              :  !
      83              :  ! The test does not seem necessary; IF a process has G=(0,0,0)
      84              :  ! (namely, the master process), then it will be the first G vector.
      85              :  ! If a process does not have G=(0,0,0) (ie, all the other processes),
      86              :  ! then they don't need to worry about the G-> 0 limit.
      87              :  !
      88              : 
      89          548 :  ltest = ALL(gvec(:,1) == 0)
      90              :  !ABI_CHECK(ltest,'The first G vector should be Gamma')
      91              : 
      92          137 :  igs=1
      93          137 :  if (ltest .and. normv(qpt, gmet, 'G') < tol4) then ! For small q and G=0, use the limit q-->0.
      94           92 :    vc_cut(1)=two_pi*rcut**2
      95           92 :    igs=2
      96              :  end if
      97       144966 :  do ig=igs,ngvec
      98       579316 :    qpg = normv(qpt(:) + gvec(:,ig), gmet, 'G')
      99       144966 :    vc_cut(ig) = four_pi* (one-COS(rcut*qpg)) / qpg**2
     100              :  end do
     101              : 
     102          137 : end subroutine cutoff_sphere
     103              : !!***
     104              : 
     105              : end module m_cutoff_sphere
     106              : !!***