A NEW APPROXIMATION IN THE FUNCTIONAL INTEGRAL APPROACH - BEYOND THE FIRST-HARMONIC APPROXIMATION

The single cross diagram approximation (SCD), beyond the first harmoni c approximation (FHA) in the functional integral approach (FIA), is re alized for the first time. The exact relation proved by Xianxi Dai [J. Phys. Condens. Matter 3 (1991) 4389; 4 (1992) 1339] and a complete in tegral representation of the psi, function turn out to be very useful in the numerical calculation. The numerical results for the symmetric Anderson model show that the SCD is better than the FHA in the medium temperature region for small and medium Coulomb interaction U. The met hod of cancelling the infinite singularities is universal in the diagr am technique in the FIA. (C) 1998 Elsevier Science B.V.