function out=Build_Density(eigenvecs, N);
nb = size(eigenvecs, 1);
D=zeros(nb,nb);
for n= 1:nb
    for m= 1:nb
        for i = 1:N/2
            D(n,m)= D(n,m) + eigenvecs(n,i)*eigenvecs(m,i);
        end
    end
end
out = D;
