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! This file is part of xtb.
!
! Copyright (C) 2017-2020 Stefan Grimme
!
! xtb is free software: you can redistribute it and/or modify it under
! the terms of the GNU Lesser General Public License as published by
! the Free Software Foundation, either version 3 of the License, or
! (at your option) any later version.
!
! xtb is distributed in the hope that it will be useful,
! but WITHOUT ANY WARRANTY; without even the implied warranty of
! MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
! GNU Lesser General Public License for more details.
!
! You should have received a copy of the GNU Lesser General Public License
! along with xtb. If not, see <https://www.gnu.org/licenses/>.
module xtb_grad_core
use xtb_mctc_accuracy, only : wp
use xtb_mctc_convert, only : evtoau
use xtb_xtb_data
use xtb_mctc_la
!! ========================================================================
! we use the complete xtb_scc_core, so we inherit all interfaces and all
! parameters. Therefore, we don't need to declare them again, note that
! by including the xtb_grad_core you also inherit the xtb_scc_core!
use xtb_scc_core
implicit none
integer, private, parameter :: mmm(20)=(/1,2,2,2,3,3,3,3,3,3,4,4,4,4,4,4,4,4,4,4/)
contains
!! ========================================================================
! derivative of S(R) enhancement factor
!! ========================================================================
pure subroutine dshellPoly(iPoly,jPoly,iRad,jRad,rab2,xyz1,xyz2,rf,dxyz)
!$acc routine seq
real(wp), intent(in) :: iPoly,jPoly
real(wp), intent(in) :: iRad,jRad
real(wp), intent(in) :: rab2
real(wp), intent(out) :: dxyz(3),rf
real(wp), intent(in) :: xyz1(3),xyz2(3)
real(wp) :: rab,k1,k2,rr,r,a,dum,rf1,rf2,dx,dy,dz
real(wp) :: t14,t15,t17,t22,t20,t23,t10,t11,t35,t13
a=0.5 ! R^a dependence 0.5 in GFN1
dx=xyz1(1)-xyz2(1)
dy=xyz1(2)-xyz2(2)
dz=xyz1(3)-xyz2(3)
rab=sqrt(rab2)
! this sloppy conv. factor has been used in development, keep it
r=iRad+jRad
rr=rab/r
k1=iPoly*0.01_wp
k2=jPoly*0.01_wp
t14 = rr**a
t15 = k1*t14
t17 = 1.0_wp/rab2
t22 = rr**a
t23 = k2*t22
rf=(1.0_wp+t15)*(1.0_wp+k2*t22)
dxyz(:)=(t15*(1.0_wp+t23)+(1.0_wp+t15)*k2*t22)*a*t17*[dx,dy,dz]
end subroutine dshellPoly
end module xtb_grad_core
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