double precision function potential(r)
  implicit none
  double precision, intent(in) :: r(3)

  double precision             :: distance

  distance = dsqrt( r(1)*r(1) + r(2)*r(2) + r(3)*r(3) )

  if (distance > 0.d0) then
     potential = -1.d0 / distance
  else
     stop 'potential at r=0.d0 diverges'
  end if

end function potential

double precision function psi(a, r)
  implicit none
  double precision, intent(in) :: a, r(3)

  psi = dexp(-a * dsqrt( r(1)*r(1) + r(2)*r(2) + r(3)*r(3) ))
end function psi

double precision function kinetic(a,r)
  implicit none
  double precision, intent(in) :: a, r(3)

  double precision             :: distance

  distance = dsqrt( r(1)*r(1) + r(2)*r(2) + r(3)*r(3) ) 

  if (distance > 0.d0) then

     kinetic =  a * (1.d0 / distance - 0.5d0 * a)

  else
     stop 'kinetic energy diverges at r=0'
  end if

end function kinetic

double precision function e_loc(a,r)
  implicit none
  double precision, intent(in) :: a, r(3)

  double precision, external :: kinetic
  double precision, external :: potential

  e_loc = kinetic(a,r) + potential(r)

end function e_loc

subroutine drift(a,r,b)
  implicit none
  double precision, intent(in)  :: a, r(3)
  double precision, intent(out) :: b(3)

  double precision :: ar_inv

  ar_inv = -a / dsqrt(r(1)*r(1) + r(2)*r(2) + r(3)*r(3))
  b(:)   = r(:) * ar_inv

end subroutine drift