Approaching the critical region of two-dimensionalφ4quantum field theory with post-Gaussian approximations

We investigate the vacuum state of (1+1)-dimensional ${\phi }^4$ quantum field theory utilizing a modification of the powerful coupled cluster method by the additional maximum-overlap condition. This permits us to construct the ground state of that field theory for nearly all values of the coupling strength. Only a small region has to be excluded where our method still fails. This is most probably due to critical behavior showing up in a change of the order parameter of the model. Our procedure predicts a behavior of the (${\phi }^4$ ${)}_2$ model in complete agreement with some rigorous mathematical statements which is not possible in the case of a Gaussian approximation only. Perhaps somewhat unexpectedly, the symmetry-breaking Hamiltonian does not have any critical point.