
! Copyright (C) 2002-2005 J. K. Dewhurst, S. Sharma and C. Ambrosch-Draxl.
! This file is distributed under the terms of the GNU Lesser General Public
! License. See the file COPYING for license details.

!BOP
! !ROUTINE: sdelta_mp
! !INTERFACE:
real(8) function sdelta_mp(n,x)
! !INPUT/OUTPUT PARAMETERS:
!   n : order (in,integer)
!   x : real argument (in,real)
! !DESCRIPTION:
!   Returns the smooth approximation to the Dirac delta function of order $N$
!   given by Methfessel and Paxton, {\it Phys. Rev. B} {\bf 40}, 3616 (1989),
!   $$ \tilde\delta(x)=\sum_{i=0}^N \frac{(-1)^i}{i!4^n\sqrt\pi} H_{2i}(x)
!    e^{-x^2},$$
!   where $H_j$ is the $j$th-order Hermite polynomial. This function has the
!   property
!   $$ \int_{-\infty}^{\infty}\tilde\delta(x)P(x)=P(0), $$
!   where $P(x)$ is any polynomial of degree $2N+1$ or less. The case $N=0$
!   corresponds to Gaussian smearing. This procedure is numerically stable
!   and accurate to near machine precision for $N\le 10$.
!
! !REVISION HISTORY:
!   Created April 2003 (JKD)
!EOP
!BOC
implicit none
! arguments
integer, intent(in) :: n
real(8), intent(in) :: x
! local variables
integer i
real(8), parameter :: sqpi=1.7724538509055160273d0
real(8) sm,t0,t1
! external functions
real(8), external :: factnm,hermite
if (n.eq.0) then
  sdelta_mp=exp(-x**2)/sqpi
  return
end if
if (n.lt.0) then
  write(*,*)
  write(*,'("Error(sdelta_mp): n < 0 : ",I8)') n
  write(*,*)
  stop
end if
if (n.gt.10) then
  write(*,*)
  write(*,'("Error(sdelta_mp): n out of range : ",I8)') n
  write(*,*)
  stop
end if
if (abs(x).gt.12.d0) then
  sdelta_mp=0.d0
  return
end if
t0=exp(-x**2)/sqpi
sm=t0
do i=1,n
  t1=t0/(factnm(i,1)*dble(4**i))
  if (mod(i,2).ne.0) t1=-t1
  sm=sm+t1*hermite(2*i,x)
end do
sdelta_mp=sm
end function
!EOC

