Cf. Wohler et E. Shuryak, 2-LOOP CORRECTION TO THE INSTANTON DENSITY FOR THE DOUBLE-WELL POTENTIAL, Physics letters. Section B, 333(3-4), 1994, pp. 467-470
Feynman diagrams in the instanton background are used for the calculat
ion of the tunneling amplitude, up to the two-loops order. Some mistak
es made in the previous works are corrected. The same method is applie
d to the next-order corrections to the ground state wave function.