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!-----------------------------------------------------------------------
!
!
SUBROUTINE force(p,nterms,iterms,pos_com,dx,dy,dz,drdotdr,pmass,pquad,acc_g,option)
!
!
!-----------------------------------------------------------------------
!
!
! Subroutine to compute the force on the p particle
!
!-----------------------------------------------------------------------
USE fly_h
implicit none
! Declaration of local variables.
! -------------------------------
INTEGER(KIND=4) :: p,i, nterms,nqterms,j,n,iqin
INTEGER(KIND=4), DIMENSION (maxnterm) :: qindex, smindex
INTEGER(KIND=4), DIMENSION (ndim) ::i_mesh,ip,csign
REAL(KIND=8) :: drdeldrg,phsm,drsm,accsm
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,acc_local,pos_s
REAL(KIND=8), DIMENSION (ndim) ::acc_local_ew, acc_local_dir,acc_local_quad
CHARACTER(LEN=4) :: option
INTEGER :: il_sh
REAL(KIND=8), INTENT(inout) :: pos_comm(:),dx(:),dy(:),dz(:),drdotdr(:)
REAL(KIND=8), INTENT(inout) :: pmass(:)
INTEGER(KIND=4), INTENT(inout) :: iterms(:)
REAL(KIND=8), INTENT(inout) ::pquad(:,:)
REAL(KIND=8), INTENT(inout) :: acc_g(:)
!=======================================================================
acc_local=0.
acc_local_ew=0.
acc_local_dir=0.
acc_local_quad=0.
!CACHE
IF(group_access.EQ.0) THEN !came from grouping
pos_l(1:ndim)=pos(1:ndim,p)
ELSE !NOT came from grouping
pos_l=pos_comm
ENDIF !came from grouping
IF(nterms.GT.maxnterm) THEN
CALL error('force error1: overflow')
ENDIF
!-----------------------------------------------------------------------
! Compute monopole contribution
!-----------------------------------------------------------------------
rpinveff=1./(0.+tiny)
epsp=eps
!-----------------------------------------------------------------------
! Loop over interaction list.
!-----------------------------------------------------------------------
DO 30 i=1,nterms
IF(group_access.EQ.0) THEN !came from grouping
il_sh=iterms(i)
IF(il_sh.gt.nbodsmax) THEN
il_sh=il_sh-nbodsmax
pos_s(1:ndim)=pos_cell(1:ndim,il_sh)
ELSE !il_sh.gt.nbodsmax
pos_s(1:ndim)=pos(1:ndim,il_sh)
ENDIF !il_sh.gt.nbodsmax
dx(i)=pos_l(1)-pos_s(1)
dy(i)=pos_l(2)-pos_s(2)
dz(i)=pos_l(3)-pos_s(3)
drdotdr(i)=dx(i)**2+dy(i)**2+dz(i)**2
ENDIF ! come from grouping
sdrdotdr=SQRT(drdotdr(i))
rinveff(i)=1./(sdrdotdr+tiny)
r3inveff(i)=rinveff(i)/(drdotdr(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(i)*r3inveff(i)
!-----------------------------------------------------------------------
! Ewald boundary periodic conditions section
!-----------------------------------------------------------------------
xx=dx(i)
yy=dy(i)
zz=dz(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(i)
acc_local_ew=acc_local_ew+f
!-----------------------------------------------------------------------
! END Ewald section
!-----------------------------------------------------------------------
30 CONTINUE
IF(option.NE.'pot ') THEN
DO 50 i=1,nterms
acc_local_dir(1)=acc_local_dir(1)-dx(i)*acci(i)
acc_local_dir(2)=acc_local_dir(2)-dy(i)*acci(i)
acc_local_dir(3)=acc_local_dir(3)-dz(i)*acci(i)
50 CONTINUE
ENDIF
!-----------------------------------------------------------------------
! If required, compute quadrupole contribution.
!-----------------------------------------------------------------------
IF(usequad) THEN
!-----------------------------------------------------------------------
! Filter out bodies.
! Compute quadrupole interaction from cells.
!-----------------------------------------------------------------------
nqterms=0
DO i=1,nterms
IF(iterms(i).GT.nbodsmax) THEN
nqterms=nqterms+1
qindex(nqterms)=i
ENDIF
ENDDO
IF(option.NE.'pot ') THEN
DO 80 i=1,nqterms
iqin=qindex(i)
quad_l(1:2*ndim-1)=pquad(1:2*ndim-1,qindex(i))
r2inveff(i)=rinveff(iqin)*rinveff(iqin)
qr5inv(i)=r3inveff(iqin)*r2inveff(i)
phiquad(i)=((-.5*((dx(iqin)**2-dz(iqin)**2)* &
quad_l(1)+(dy(iqin)**2- &
dz(iqin)**2)*quad_l(4))- &
(dx(iqin)*dy(iqin)*quad_l(2)+ &
dx(iqin)*dz(iqin)*quad_l(3)+ &
dy(iqin)*dz(iqin)*quad_l(5)))* &
qr5inv(i))*5.*r2inveff(i)
acc_local_quad(1)=acc_local_quad(1)+dx(iqin)*phiquad(i)+ &
(dx(iqin)*quad_l(1)+ &
dy(iqin)*quad_l(2)+ &
dz(iqin)*quad_l(3))*qr5inv(i)
acc_local_quad(2)=acc_local_quad(2)+dy(iqin)*phiquad(i)+ &
(dy(iqin)*quad_l(4)+ &
dx(iqin)*quad_l(2)+ &
dz(iqin)*quad_l(5))*qr5inv(i)
acc_local_quad(3)=acc_local_quad(3)+dz(iqin)*phiquad(i)+ &
(dz(iqin)*(-quad_l(1)- &
quad_l(4))+dx(iqin)* &
quad_l(3)+dy(iqin)* &
quad_l(5))*qr5inv(i)
80 CONTINUE
ENDIF
ENDIF
acc_local=acc_local_ew+acc_local_dir+acc_local_quad
IF(group_access.EQ.0) THEN !access from grouping section
acc_local(1)=acc_local(1)+acc_g(1)
acc_local(2)=acc_local(2)+acc_g(2)
acc_local(3)=acc_local(3)+acc_g(3)
ENDIF
IF(rmt_acc) THEN
acc_rmt(1:ndim,p)=acc_rmt(1:ndim,p)+acc_local(1:ndim)
ELSE
acc(1:ndim,p)=acc_local(1:ndim)
ENDIF
RETURN
END