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!-----------------------------------------------------------------------
!
!
SUBROUTINE force_group(p,nterms_gr,iterms_gr,drdotdr_gr,,dx_gr,dy_gr,dz_gr,pmass_gr,acc_g,pquad_gr,option)
!
!
!-----------------------------------------------------------------------
!
!
! Subroutine to compute the force F_far componete in a cell-group
!
!
!-----------------------------------------------------------------------
USE fly_h
implicit none
! Declaration of local variables.
! -------------------------------
INTEGER(KIND=4) :: p,i, nterms_gr,nqterms,j,n,it,iqin
INTEGER(KIND=4), DIMENSION (maxnterm) :: qindex, smindex
INTEGER(KIND=4), DIMENSION (ndim) ::i_mesh,ip,csign
INTEGER :: il_sh
REAL(KIND=8) :: drdeldrg,phsm,drsm,accsm,mass_s
REAL(KIND=8) :: sdrdotdr, epsp,epsi,rpinveff
REAL(KIND=8) :: xx,yy,zz,wt1,wt2,wt3,wt4,wt5,wt6,wt7,wt8
REAL(KIND=8) :: wi1,wj1,wk1,wi2,wj2,wk2
REAL(KIND=8), DIMENSION (maxnterm) ::r3inveff,rinveff,qr5inv,phiquad
REAL(KIND=8), DIMENSION (maxnterm) :: r2inveff,acci
REAL(KIND=8), DIMENSION (1:2*ndim-1) ::quad_l
REAL(KIND=8), DIMENSION (ndim) ::pos_l,r_abs,f
INTEGER(KIND=4), INTENT(inout) :: iterms_gr(:)
REAL(KIND=8), INTENT(inout) ::drdotdr_gr(:),dx_gr(:),dy_gr(:),dz_gr(:)
REAL(KIND=8), INTENT(inout) :: pmass_gr(:)
REAL(KIND=8), INTENT(inout) :: acc_g(:)
REAL(KIND=8), INTENT(inout) :: pquad_gr(:,:)
CHARACTER(LEN=4) :: option
!-----------------------------------------------------------------------
il_sh=p-nbodsmax
pos_l(1:ndim)=pos_cell(1:ndim,il_sh)
!-----------------------------------------------------------------------
! Compute monopole contribution; temporarily set mass of body p to
! zero to avoid possible self-interaction contribution.
!-----------------------------------------------------------------------
rpinveff=1./(0.+tiny)
epsp=eps
!-----------------------------------------------------------------------
! Loop over interaction list.
!-----------------------------------------------------------------------
DO 30 i=1,nterms_gr
it=iterms_gr(i)
sdrdotdr=SQRT(drdotdr_gr(i))
rinveff(i)=1./(sdrdotdr+tiny)
r3inveff(i)=rinveff(i)/(drdotdr_gr(i)+tiny)
epsi=eps
drdeldrg=sdrdotdr*ninterp/(epsp+epsi)
smindex(i)=drdeldrg
smindex(i)=MIN(ninterp,smindex(i))
drsm=MIN(one,drdeldrg-smindex(i))
phsm=(1.-drsm)*phsmooth(smindex(i))+ &
drsm*phsmooth(1+smindex(i))
accsm=(1.-drsm)*acsmooth(smindex(i))+ &
drsm*acsmooth(1+smindex(i))
rinveff(i)=phsm*rinveff(i)
r3inveff(i)=accsm*r3inveff(i)
acci(i)=pmass_gr(i)*r3inveff(i)
!-----------------------------------------------------------------------
! Ewald boundary periodic conditions section
!-----------------------------------------------------------------------
xx=dx_gr(i)
yy=dy_gr(i)
zz=dz_gr(i)
r_abs(1)=abs(xx)
r_abs(2)=abs(yy)
r_abs(3)=abs(zz)
if(r_abs(1).eq.0.0) then
csign(1)=0
else
csign(1)=xx/r_abs(1)
endif
if(r_abs(2).eq.0.0) then
csign(2)=0.
else
csign(2)=yy/r_abs(2)
endif
if(r_abs(3).eq.0.0) then
csign(3)=0.
else
csign(3)=zz/r_abs(3)
endif
i_mesh=r_abs*linv
ip=i_mesh+1
wi1=1.-linv*(r_abs(1)-rk(i_mesh(1),1))
wi2=1.-linv*(rk(ip(1),1)-r_abs(1))
wj1=1.-linv*(r_abs(2)-rk(i_mesh(2),2))
wj2=1.-linv*(rk(ip(2),2)-r_abs(2))
wk1=1.-linv*(r_abs(3)-rk(i_mesh(3),3))
wk2=1.-linv*(rk(ip(3),3)-r_abs(3))
wt1=wi1*wj1*wk1
wt2=wi2*wj1*wk1
wt3=wi1*wj2*wk1
wt4=wi1*wj1*wk2
wt5=wi2*wj2*wk1
wt6=wi2*wj1*wk2
wt7=wi1*wj2*wk2
wt8=wi2*wj2*wk2
f(1:3)=wt1*fc(i_mesh(1),i_mesh(2),i_mesh(3),1:3)
f(1:3)=f(1:3)+wt2*fc(ip(1),i_mesh(2),i_mesh(3),1:3)
f(1:3)=f(1:3)+wt3*fc(i_mesh(1),ip(2),i_mesh(3),1:3)
f(1:3)=f(1:3)+wt4*fc(i_mesh(1),i_mesh(2),ip(3),1:3)
f(1:3)=f(1:3)+wt5*fc(ip(1),ip(2),i_mesh(3),1:3)
f(1:3)=f(1:3)+wt6*fc(ip(1),i_mesh(2),ip(3),1:3)
f(1:3)=f(1:3)+wt7*fc(i_mesh(1),ip(2),ip(3),1:3)
f(1:3)=f(1:3)+wt8*fc(ip(1),ip(2),ip(3),1:3)
f=csign*f*pmass_gr(i)
acc_g(1)=acc_g(1)+f(1)
acc_g(2)=acc_g(2)+f(2)
acc_g(3)=acc_g(3)+f(3)
!-----------------------------------------------------------------------
! END Ewald section
!-----------------------------------------------------------------------
30 CONTINUE
IF(option.NE.'pot ') THEN
DO 50 i=1,nterms_gr
acc_g(1)=acc_g(1)-dx_gr(i)*acci(i)
acc_g(2)=acc_g(2)-dy_gr(i)*acci(i)
acc_g(3)=acc_g(3)-dz_gr(i)*acci(i)
50 CONTINUE
ENDIF
!-----------------------------------------------------------------------
! If required, compute quadrupole contribution.
!-----------------------------------------------------------------------
IF(usequad) THEN
!-----------------------------------------------------------------------
! Filter out bodies.
!-----------------------------------------------------------------------
nqterms=0
DO i=1,nterms_gr
IF(iterms_gr(i).GT.nbodsmax) THEN
nqterms=nqterms+1
qindex(nqterms)=i
ENDIF
ENDDO
IF(option.NE.'pot ') THEN
DO i=1,nqterms
iqin=qindex(i)
quad_l(1:2*ndim-1)=pquad_gr(1:2*ndim-1,iqin)
r2inveff(i)=rinveff(iqin)*rinveff(iqin)
qr5inv(i)=r3inveff(iqin)*r2inveff(i)
phiquad(i)=((-.5*((dx_gr(iqin)**2-dz_gr(iqin)**2)* &
quad_l(1)+(dy_gr(iqin)**2- &
dz_gr(iqin)**2)*quad_l(4))- &
(dx_gr(iqin)*dy_gr(iqin)*quad_l(2)+ &
dx_gr(iqin)*dz_gr(iqin)*quad_l(3)+ &
dy_gr(iqin)*dz_gr(iqin)*quad_l(5)))* &
qr5inv(i))*5.*r2inveff(i)
acc_g(1)=acc_g(1)+dx_gr(iqin)*phiquad(i)+ &
(dx_gr(iqin)*quad_l(1)+ &
dy_gr(iqin)*quad_l(2)+ &
dz_gr(iqin)*quad_l(3))*qr5inv(i)
acc_g(2)=acc_g(2)+dy_gr(iqin)*phiquad(i)+ &
(dy_gr(iqin)*quad_l(4)+ &
dx_gr(iqin)*quad_l(2)+ &
dz_gr(iqin)*quad_l(5))*qr5inv(i)
acc_g(3)=acc_g(3)+dz_gr(iqin)*phiquad(i)+ &
(dz_gr(iqin)*(-quad_l(1)- &
quad_l(4))+dx_gr(iqin)* &
quad_l(3)+dy_gr(iqin)* &
quad_l(5))*qr5inv(i)
ENDDO
ENDIF !IF(option.NE.'pot ')
ENDIF ! IF(usequad)
RETURN
END