field at these investigated frequencies, and therefore if the investigated system can possess nonzero gyrotropy coefficients (see Eq.(\[eq33\])).

In this effective Hamiltonian method, developed in Ref.[@Walizer2006] for (Ba,Sr)TiO$_3$ (BST) compounds, the degrees of freedom are: the local mode vectors in each 5-atom unit cell (these local modes are directly proportional to the electric dipoles in these cells), the homogeneous strain tensor and inhomogeneous-strain-related variables [@Zhong1995]. The total internal energy contains a local mode self-energy, short-range and long-range interactions between local modes, an elastic energy and interactions between local modes and strains. Further energetic terms model the effect of the interfaces between the wires and the medium on electric dipoles and strains, as well as take into account the strain that is induced by the size difference between Ba and Sr ions and its effect on physical properties. The parameters entering the total internal energy are derived from first principles. This $H_{\rm{eff}}$ can be used within Monte-Carlo or Molecular dynamics simulations to obtain finite-temperature static or dynamical properties, respectively, of relatively large supercells (i.e., of the order of thousands or tens of thousands of atoms). Previous calculations [@Choudhury2011; @Walizer2006; @Lisenkov2007; @Hlinka2008; @Quingteng2010] for various disordered or ordered BST systems demonstrated the 