TESTING RNTGRL
  k=power of exp(-x) in integral from 0 to infty
  n=no of points
  ans=int 4*pi*r*r* exp(-k*r)*(k^3/8*pi)-1
 k= 1 n= 20 ans=  3.32891024E-05
 k= 1 n= 40 ans=  2.98962104E-05
 k= 1 n= 60 ans=  1.46417051E-07
 k= 1 n= 80 ans= -5.6315097E-10
 
 k= 2 n= 20 ans=  6.76535326E-05
 k= 2 n= 40 ans= -8.2802101E-08
 k= 2 n= 60 ans= -1.79164044E-08
 k= 2 n= 80 ans= -5.88331728E-09
 
 k= 3 n= 20 ans= -3.86765252E-06
 k= 3 n= 40 ans= -2.11181624E-07
 k= 3 n= 60 ans= -3.84670488E-08
 k= 3 n= 80 ans= -1.17741443E-08
 
 k= 4 n= 20 ans= -7.69154538E-06
 k= 4 n= 40 ans= -1.98264512E-07
 k= 4 n= 60 ans= -2.42356518E-08
 k= 4 n= 80 ans= -5.53089519E-09
 
 k= 5 n= 20 ans=  8.21752155E-07
 k= 5 n= 40 ans=  4.18155181E-07
 k= 5 n= 60 ans=  1.08926201E-07
 k= 5 n= 80 ans=  3.86302208E-08