# Phonon non-analytical term correction

The non-analytical term correction (NAC) corrects the dynamical matrices using (Baroni, 2001, p527; Zhang et al., 2022),

where the $Z^*$ denotes the Born effective charge and $\epsilon^\infty$ denotes the dielectric constant.

In phonopy, this is achieved by performing a separate VASP run with LEPSILON = .TRUE. and a denser $k$-mesh to calculate the $Z^*$ and $\epsilon^\infty$, then use phonopy-vasp-born to post-process and create a BORN file.