ADAS Subroutine r8p

      FUNCTION R8P ( N , L )
C
C
C-----------------------------------------------------------------------
C
C  **************** FORTRAN77 REAL*8 FUNCTION: R8P    ******************
C
C  PURPOSE: EXPECTATION VALUE <1/R^4> IN HYDROGEN APPROXIMATION FOR USE
C           IN DIPOLE POLARISABILITY ENERGY SHIFT EVALUATION
C
C  CALLING PROGRAM: CXLTHE, CXSODE
C
C  FUNC:   (R*8)  R8P     = EXPECTATION VALUE <1/R^4>
C
C  INPUT:  (I*4)  N       = PRINCIPAL QUANTUM NUMBER.
C  INPUT:  (I*4)  L       = ORBITAL QUANTUM NUMBER.
C
C          (R*8)  XN      = REAL VALUE = N.
C          (R*8)  XL      = REAL VALUE = L.
C
C ROUTINES: NONE
C
C AUTHOR:  JONATHAN NASH (TESSELLA SUPPORT SERVICES PLC)
C          K1/0/81
C          JET EXT. 5183
C
C DATE:    02/11/93
C
C VERSION: 1.1                          DATE: 02-11-093
C MODIFIED: JONATHAN NASH
C               - FIRST RELEASE
C
C VERSION: 1.2                          DATE: 17-04-07
C MODIFIED: HUGH SUMMERS
C               - FIRST FULLY COMMENTED RELEASE
C
C-----------------------------------------------------------------------
C
C-----------------------------------------------------------------------
      INTEGER             L,           N