Effect of Anharmonicity on the WKB Energy Splitting in a Double Well   Potential
  We investigate the effect of anharmonicity on the WKB approximation in a double well potential. By incorporating the anharmonic perturbation into the WKB energy splitting formula we show that the WKB approximation can be greatly improved in the region over which the tunneling is appreciable. We also observe that the usual WKB results can be obtained from our formalism as a limiting case in which the two potential minima are far apart. 
Chang Soo Park, Soo-Young Lee, Jae-Rok Kahng, Sahng-Kyoon Yoo, D.K.   Park, C.H. Lee, Eui-Soon Yim