function [Atilde b] = BuildSystem(nv,A,QB,nB,Pref,natm)
  Atilde = A;
  b = zeros(nv,1);
  Atilde(natm,:) = 0.0;
  Atilde(natm,natm) = 1.0;
  b(natm) = Pref;
  b(nB) = QB; 
end