mirror of https://gitlab.com/QEF/q-e.git
174 lines
5.1 KiB
Fortran
174 lines
5.1 KiB
Fortran
!
|
|
! Copyright (C) 2001 PWSCF group
|
|
! This file is distributed under the terms of the
|
|
! GNU General Public License. See the file `License'
|
|
! in the root directory of the present distribution,
|
|
! or http://www.gnu.org/copyleft/gpl.txt .
|
|
!
|
|
!
|
|
!-----------------------------------------------------------------------
|
|
subroutine force_ew (alat, nat, ntyp, ityp, zv, at, bg, tau, &
|
|
omega, g, gg, ngm, gstart, gamma_only, gcutm, strf, forceion)
|
|
!-----------------------------------------------------------------------
|
|
!
|
|
! This routine computes the Ewald contribution to the forces,
|
|
! both the real- and reciprocal-space terms are present
|
|
!
|
|
#include "f_defs.h"
|
|
USE kinds
|
|
implicit none
|
|
!
|
|
! First the dummy variables
|
|
!
|
|
|
|
integer :: nat, ntyp, ngm, ityp (nat), gstart
|
|
! input: the number of atoms
|
|
! input: the number of types of atom
|
|
! input: the number of G vectors
|
|
! input: the type of each atom
|
|
! input: first non-zero G vector
|
|
logical :: gamma_only
|
|
|
|
real(kind=DP) :: factor, tau (3, nat), g (3, ngm), gg (ngm), zv (ntyp), &
|
|
at (3, 3), bg (3, 3), omega, gcutm, alat
|
|
! input: the coordinates of the atoms
|
|
! input: the G vectors
|
|
! input: the moduli of G vectors
|
|
! input: the charge of the atoms
|
|
! input: the direct lattice vectors
|
|
! input: the reciprocal lattice vectors
|
|
! input: the volume of the unit cell
|
|
! input: cut-off of g vectors
|
|
! input: the edge of the cell
|
|
!
|
|
complex(kind=DP) :: strf (ngm, ntyp)
|
|
! input: the structure factor on the potential
|
|
!
|
|
real(kind=DP) :: forceion (3, nat)
|
|
! output: the ewald part of the forces
|
|
!
|
|
integer, parameter :: mxr=50
|
|
! the maximum number of R vectors
|
|
|
|
real(kind=DP), parameter :: e2=2.d0, tpi = 2.d0 * 3.14159265358979d0
|
|
! the square of the electron charge
|
|
! two times pi
|
|
|
|
integer :: ig, n, na, nb, nt, nrm, ipol
|
|
! counter on G vectos
|
|
! counter on r vectors
|
|
! counter on atoms
|
|
! counter on atoms
|
|
! counter on atomic types
|
|
! the number of R vectors for real space su
|
|
! counter on polarization
|
|
|
|
real(kind=DP) :: sumnb, arg, tpiba2, alpha, dtau (3), r (3, mxr), &
|
|
r2 (mxr), erfc, rmax, rr, charge, upperbound, fact
|
|
! auxiliary variable for speed
|
|
! the argument of the exponential
|
|
! 2 pi /alat
|
|
! the alpha parameter
|
|
! the difference of two tau
|
|
! the position of the atoms in the shell
|
|
! the square of r
|
|
! the complementary error function
|
|
! the maximum r
|
|
! the modulus of the r vectors
|
|
! the total charge
|
|
! used to determine alpha
|
|
|
|
complex(kind=DP), allocatable :: aux (:)
|
|
! auxiliary space
|
|
!
|
|
forceion(:,:) = 0.d0
|
|
tpiba2 = (tpi / alat) **2
|
|
charge = 0.d0
|
|
do na = 1, nat
|
|
charge = charge+zv (ityp (na) )
|
|
enddo
|
|
!
|
|
! choose alpha in order to have convergence in the sum over G
|
|
! upperbound is a safe upper bound for the error ON THE ENERGY
|
|
!
|
|
alpha = 1.1d0
|
|
10 alpha = alpha - 0.1d0
|
|
if (alpha.eq.0.d0) call errore ('force_ew', 'optimal alpha not foun &
|
|
&d', 1)
|
|
upperbound = e2 * charge**2 * sqrt (2.d0 * alpha / tpi) * erfc ( &
|
|
sqrt (tpiba2 * gcutm / 4.d0 / alpha) )
|
|
if (upperbound.gt.1.0d-6) goto 10
|
|
!
|
|
! G-space sum here
|
|
!
|
|
allocate(aux(ngm))
|
|
aux(:) = (0.d0, 0.d0)
|
|
|
|
do nt = 1, ntyp
|
|
do ig = gstart, ngm
|
|
aux (ig) = aux (ig) + zv (nt) * conjg (strf (ig, nt) )
|
|
enddo
|
|
enddo
|
|
do ig = gstart, ngm
|
|
aux (ig) = aux (ig) * exp ( - gg (ig) * tpiba2 / alpha / 4.d0) &
|
|
/ (gg (ig) * tpiba2)
|
|
enddo
|
|
if (gamma_only) then
|
|
fact = 4.d0
|
|
else
|
|
fact = 2.d0
|
|
end if
|
|
do na = 1, nat
|
|
do ig = gstart, ngm
|
|
arg = tpi * (g (1, ig) * tau (1, na) + g (2, ig) * tau (2, na) &
|
|
+ g (3, ig) * tau (3, na) )
|
|
sumnb = cos (arg) * DIMAG (aux(ig)) - sin (arg) * DREAL (aux(ig) )
|
|
forceion (1, na) = forceion (1, na) + g (1, ig) * sumnb
|
|
forceion (2, na) = forceion (2, na) + g (2, ig) * sumnb
|
|
forceion (3, na) = forceion (3, na) + g (3, ig) * sumnb
|
|
enddo
|
|
do ipol = 1, 3
|
|
forceion (ipol, na) = - zv (ityp (na) ) * fact * e2 * tpi**2 / &
|
|
omega / alat * forceion (ipol, na)
|
|
enddo
|
|
enddo
|
|
deallocate (aux)
|
|
if (gstart.eq.1) goto 100
|
|
!
|
|
! R-space sum here (only for the processor that contains G=0)
|
|
!
|
|
rmax = 5.d0 / (sqrt (alpha) * alat)
|
|
!
|
|
! with this choice terms up to ZiZj*erfc(5) are counted (erfc(5)=2x10^-1
|
|
!
|
|
do na = 1, nat
|
|
do nb = 1, nat
|
|
if (nb.eq.na) goto 50
|
|
do ipol = 1, 3
|
|
dtau (ipol) = tau (ipol, na) - tau (ipol, nb)
|
|
enddo
|
|
!
|
|
! generates nearest-neighbors shells r(i)=R(i)-dtau(i)
|
|
!
|
|
call rgen (dtau, rmax, mxr, at, bg, r, r2, nrm)
|
|
do n = 1, nrm
|
|
rr = sqrt (r2 (n) ) * alat
|
|
factor = zv (ityp (na) ) * zv (ityp (nb) ) * e2 / rr**2 * &
|
|
(erfc (sqrt (alpha) * rr) / rr + &
|
|
sqrt (8.0d0 * alpha / tpi) * exp ( - alpha * rr**2) ) * alat
|
|
do ipol = 1, 3
|
|
forceion (ipol, na) = forceion (ipol, na) - factor * r (ipol, n)
|
|
enddo
|
|
enddo
|
|
50 continue
|
|
enddo
|
|
enddo
|
|
100 continue
|
|
#ifdef __PARA
|
|
|
|
call reduce (3 * nat, forceion)
|
|
#endif
|
|
return
|
|
end subroutine force_ew
|
|
|