  The general eight-vertex model on a square lattice is studied numerically by
using the Corner Transfer Matrix Renormalization Group method. The method is
tested on the symmetric (zero-field) version of the model, the obtained
dependence of critical exponents on model's parameters is in agreement with
Baxter's exact solution and weak universality is verified with a high accuracy.
It was suggested longtime ago that the symmetric eight-vertex model is a
special exceptional case and in the presence of external fields the
eight-vertex model falls into the Ising universality class. We confirm
numerically this conjecture in a subspace of vertex weights, except for two
specific combinations of vertical and horizontal fields for which the system
still exhibits weak universality.
