Table[
      FindMinimum[
        9/(8*R1^2) + 2/R2^2 - 3*Z/(2*R1) - 
          Z/R2 + ((R1/R2)^3 - 4.8*(R1/R2)^2 + 32)/(32*
                R2),  {R1, 0.8}, {R2, 4.0}],  {Z, 2, 
        15}]