Potential energy function describing the interaction between two atoms of a diatomic molecule is $U(r)=a/r^{12} - b/r^6$.Force acting between them will be zero when the distance between the would be?

Question

options are

$(2a/b)^{1/6}

(b/2a)^{1/6}

(a/b)^{1/6}

(b/a)^{1/6}.$