  We analyze the onset of incommensurabilities around the VBS point of the S=1
bilinear-biquadratic model. We propose a simple effective field theory which is
capable of reproducing all known properties of the commensurate-incommensurate
transition at the disorder point $\theta_{\rm vbs}$. Moreover, the theory
predicts another special point $\theta_{\rm disp}$, distinct from the VBS
point, where the Haldane gap behaves singularly. The ground state energy
density is an analytic function of the model parameters everywhere, thus we do
not have phase transitions in the conventional sense.

Strongly Correlated Electrons;Statistical Mechanics