(* S 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+6.75*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+3.0*2.0*sig3/R3+16/(R1+R2)+24/((R1+R2)*Sqrt[8/3])+
    12/a+36/(R2+R3)+12/(R1+R2+a)+13*Sqrt[3/8]/a /. c;
Vne=-3.0*z/R1-8*z/(R1+R2)-6*z/a;

func = T + Vee + Vne;

t = Table[FindMinimum[func, {R1,0.07}, {R2,0.2}, {R3,0.5}, {MaxIterations -> 500}], {z,16,26}]

Table[N[Vne /. c /. t[[{z-15},2]],10], {z,16,26}]
Table[N[Vee /. c /. t[[{z-15},2]],10], {z,16,26}]
Table[N[-(Vee+Vne)/T /. c /. t[[{z-15},2]],10], {z,16,26}]
Table[N[a /. c /. t[[{z-15},2]],10], {z,16,26}]