(* CH4 Molekül nach Kimball, parametrisiert
   mit G2//6-311g(Propan) V-terme automatisch mit alkan.sys *)

Clear[k0,k2,sig0,sig2,u,c]; n = 1.;  z = 6.0; z2 = z*z;
(* Parameterliste c, angepasst für Propan an G2//6-311g *)
 c = {k0 -> 1.02246687, k2 -> 1.20537762,
     sig0 -> 0.30582536, sig2 -> 0.35441063};
(* alte Parameterliste wie in alkanr.pas *)
 (*  c = {k0 -> 1.02093019, k2 -> 1.21061216,
     sig0 -> 0.30488118, sig2 -> 0.35047573}; *)

  (* Terme automatisch mit ChemEdu/Kimball/Alkan.sys *)

T = 2.25*n*k0/P^2+4.5*(n+1.0)*k2/R^2 /. c ;

p = X;     p2 = p*p;   r = P+R; 
r2 = r*r;  r2p2 = r2*p2;  r2p = r2*p; 
ad = 8./3.;   zd = 2./3.;

  Vee=3.0*n*sig0/P+
  (n+1.0)*6.0*sig2/R+
  16/r+
  24/Sqrt[ad*r2];

  Vne=-3.0*n*z/P-2.0*(n+1.0)*(3.0-((p-1.0)*(1.0+P/R))^2)/R-
  24/Sqrt[zd*r2p+r2p2+r2]-
  8*z/r-
  8/(r*p);

  Vnn=4*z/(r*p)+
  6/Sqrt[ad*r2p2];
      
func = T + Vee + Vne + Vnn /. c;

N[t = FindMinimum[func, {P, 0.26112},{R, 1.2769},
                        {X, 1.3559}], 10]

u = t[[2]];

N[T /. u,10]

N[Vne /. u,10]

N[Vee /. u /. c, 10]

N[Vnn /. u, 10]

dch = 0.529177*X*(P+R) /. u      (* C-H Abstand *)

N[-(Vne+Vee+Vnn)/T /. u /. c, 8] (* Virial Theorem *)
(* Next: -E(CH4) - n*|C| - (2n+2)*0.5 *)
(-t[[1]] - n*37.784301 - n - 1.)*627.50956
(* Atomisierungsenergie bei 0 K in kcal/mol *)