CENTRAL CHARGES OF THE 6-VERTEX AND 19-VERTEX MODELS WITH TWISTED BOUNDARY-CONDITIONS

A new and general analytic method for calculating finite-size corrections and central charges is applied to the 6- and 19-vertex models and their related spin-1/2 and spin-1 XXZ chains with twisted boundary conditions. Nonlinear integral equations are derived from which the central charge c can be extracted in terms of Rogers dilogarithms. For twist angle-phi, the central charge is [GRAPHICS] where gamma-is the crossing parameter or chain anisotropy and spin S = 1/2 or 1. For periodic boundary conditions (phi = 0) this reduces to the known results c = 1 and c = 3/2, respectively.