function [chi,sigma,theta,alpha] = set_default_para_EP(xi)
theta = 1.01;
sigma = 1.1;
sigma_dd = (sigma / (2-sigma))^2;
omega = 1/sigma + xi / (2*sigma*sigma) + xi / (sigma*sigma);
chi = 1 + 4*omega*sigma_dd ;
alpha = (theta - 1) / ((theta+1)*(xi+2));
chi = chi +1e-12;
alpha = alpha - 1e-12;
