FindMinimum[
      2.25/Q^2 + 2.25/P^2 + 1/(r + R) - 9/Q + (r^2 - R^2)/P^3 - 
        1.2*Q^2/P^3 + 0.9/P + 0.9/Q, {P, 2}, {Q, 0.6}, {R, 1.2}, {r, 
        1}]