(* Ar atom nach Kimball, Aussen-sp-Kugeln 11.09.2012 *)
Clear[k1,k2,k3,sig1,sig2,sig3,c,z,R1,R2,R3,a];
c = {k1 -> 1.0, k2 -> 1.0, k3->1.0, sig1 -> 0.26, sig2 -> 0.3, sig3 -> 0.3}; (* <<<<<<<<<<<< *)

T = 2.25*k1/R1^2+9.0*k2/R2^2+9.0*k3/R3^2 /. c;
a = (R1+R2)/3+Sqrt[(R2+R3)^2-8*(R1+R2)^2/9];
Vee = 3.0*sig1/R1+4.0*3.0*sig2/R2+4.0*3.0*sig3/R3+16/(R1+R2)+24*Sqrt[3/8]/(R1+R2)+
      16/a+48/(R2+R3)+16/(R1+R2+a)+24*Sqrt[3/8]/a /. c;
Vne=-3.0*z/R1-8*z/(R1+R2)-8*z/a;

func = T + Vee + Vne;

t = Table[FindMinimum[func, {R1,0.07}, {R2,0.25}, {R3,0.7}, {MaxIterations -> 500}], {z,18,23}]

Table[N[Vne /. c /. t[[{z-17},2]],10], {z,18,23}]
Table[N[Vee /. c /. t[[{z-17},2]],10], {z,18,23}]
Table[N[-(Vee+Vne)/T /. c /. t[[{z-17},2]],10], {z,18,23}]
Table[N[a /. c /. t[[{z-17},2]],10], {z,18,23}]