When a is negative, the potential corresponds to a double-well potential; when a is zero, there is no second order term and the potential is quartic; and when a is positive, it is a symmetric single well potential with anharmonicity coming from the quartic bx 4 term. In the double well region where a is negative, the barrier decreases as a becomes less negative, making the 0 → 1 tunnelling splitting increase as seen in the quantum reference.