SumMd1[x_, rho_] :=
Module[{n = 1},
  If [(x == 0),
      1-rho,
      If [(x == 1),
	  ( (1-rho) (E^rho - 1) ),
	  If [(x > 1),
	      ( (1-rho) Sum[(-1)^(x-n) E^(n rho)
	      ( (n rho)^(x-n)/(x-n)! + (n rho)^(x-n-1)/(x-n-1)! ),
	      {n,1,x}] ),
	      0
	  ]
      ]
  ]
]