(* Diamond as {[C(+4)(2e)4/2}infG}, structure of beta-Cristobalite, ES 1963/2012 *)
Clear[k1,k2,sig1,sig2,c,Z,R1,R2];
c = {k1 -> 1.02118, k2 -> 1.3513, sig1 -> 0.306, sig2 -> 0.30515}; (* Prefacts as for hydrocarbons *)
Z=6.0; M=4.4394; EC=-1030.08;

(* He and Neon shell *)
T = 2.25*k1/R1^2+4.5*k2/R2^2 /. c;
Vee=3.0*sig1/R1+6.0*sig2/R2 /. c;
Vne=-3.0*Z/R1;
(* Madelung lattice energy *)
MadE = - M*(Z-2.0)/(R1+R2);

(* Total energy, to be minimized *)
Etot = T + Vee + Vne + MadE ;

t = FindMinimum[Etot, {R1,0.26}, {R2,1.2},{Method -> Automatic},{MaxIterations -> 500}]

(* Virial Ratio *)
N[-(Vee+Vne+MadE)/T /. c /. t[[2]],10]
(* Lattice constant, angstrom *)
N[2*0.529177*(R1+R2) /. t[[2]],10]
(* Lattice energy at 0 K, eV *)
Del = t[[1]]*27.2114-EC
(* Sublimation energy at 0 K, kJ/mol, kcal/mol *)
-Del*96.48532
-Del*23.06055