error Subroutine

   subroutine error(x,Err)

      real(wp),intent(in) :: x !! Argument of `erf(x)`
      real(wp),intent(out) :: Err !! `erf(x)`

      real(wp) :: c0 , er , r , x2
      integer :: k

      real(wp),parameter :: eps = 1.0e-15_wp

      x2 = x*x
      if ( abs(x)<3.5_wp ) then
         er = 1.0_wp
         r = 1.0_wp
         do k = 1 , 50
            r = r*x2/(k+0.5_wp)
            er = er + r
            if ( abs(r)<=abs(er)*eps ) exit
         enddo
         c0 = 2.0_wp/sqrtpi*x*exp(-x2)
         Err = c0*er
      else
         er = 1.0_wp
         r = 1.0_wp
         do k = 1 , 12
            r = -r*(k-0.5_wp)/x2
            er = er + r
         enddo
         c0 = exp(-x2)/(abs(x)*sqrtpi)
         Err = 1.0_wp - c0*er
         if ( x<0.0_wp ) Err = -Err
      endif

   end subroutine error