collision_force Subroutine

  subroutine collision_force(spp,U,Fcoll)
    !! For multiple-species collisions
    !! J. R. Martin-Solis et al. PoP 22, 092512 (2015)
    !! if (params%collisions .AND. (TRIM(params%collisions_model) .EQ.
    !! 'MULTIPLE_SPECIES')) then call collision_force(spp(ii),U_os,Fcoll)
    !!	U_RC = U_RC + a*Fcoll/spp(ii)%q end if

    TYPE(SPECIES), INTENT(IN) 		:: spp
    REAL(rp), DIMENSION(3), INTENT(IN) 	:: U
    REAL(rp), DIMENSION(3), INTENT(OUT) :: Fcoll
    REAL(rp), DIMENSION(3) 		:: V
    REAL(rp), DIMENSION(3) 		:: Fcolle
    REAL(rp), DIMENSION(3) 		:: Fcolli
    REAL(rp) 				:: gamma
    REAL(rp) 				:: tmp
    REAL(rp) 				:: ae
    REAL(rp) 				:: ai
    REAL(rp) 				:: Clog_ef
    REAL(rp) 				:: Clog_eb
    REAL(rp) 				:: Clog_eH
    REAL(rp) 				:: Clog_eZj
    REAL(rp) 				:: Clog_eZo
    INTEGER 				:: ppi

    gamma = SQRT(1.0_rp + DOT_PRODUCT(U,U))
    V = U/gamma

    tmp = (gamma - 1.0_rp)*SQRT(gamma + 1.0_rp)
    Clog_ef = log(0.5_rp*tmp*(cparams_ms%rD/cparams_ms%re)/gamma)
    ae = cparams_ms%nef*Clog_ef
    do ppi=1_idef,cparams_ms%num_impurity_species
       Clog_eb = log(tmp*cparams_ms%Ee_IZj(ppi))
       ae = ae + cparams_ms%neb(ppi)*Clog_eb
    end do

    tmp = (gamma**2 - 1.0_rp)/gamma
    Clog_eH = log( tmp*(cparams_ms%rD/cparams_ms%re) )
    ai = cparams_ms%nH*Clog_eH
    do ppi=1_idef,cparams_ms%num_impurity_species
       Clog_eZj = log( cparams_ms%rD/(cparams_ms%Zj(ppi)* &
            cparams_ms%re*cparams_ms%Ee_IZj(ppi)) )
       Clog_eZo = log(tmp*cparams_ms%Ee_IZj(ppi))
       ai = ai + &
            cparams_ms%nz(ppi)*(Clog_eZj*cparams_ms%Zj(ppi)**2 + &
            Clog_eZo*cparams_ms%Zo(ppi)**2)
    end do

    tmp = gamma*(gamma + 1.0_rp)/(SQRT(DOT_PRODUCT(U,U))**3)
    Fcolle = -4.0_rp*C_PI*ae*spp%m*(cparams_ms%re**2)*tmp*U

    tmp = gamma/(SQRT(DOT_PRODUCT(U,U))**3)
    Fcolli = -4.0_rp*C_PI*ai*spp%m*(cparams_ms%re**2)*tmp*U

    Fcoll = Fcolle + Fcolli
  end subroutine collision_force