Calculation of the dielectric constant ϵ and first nonlinear susceptibility χ(2) of crystalline potassium dihydrogen phosphate by the coupled perturbed Hartree–Fock and coupled perturbed Kohn–Sham schemes as implemented in the CRYSTAL code

The high-frequency dielectric ϵ and the first nonlinear electric susceptibility χ(2) tensors of crystalline potassium dihydrogen phosphate (KH2PO4) are calculated by using the coupled perturbed Hartree–Fock and Kohn–Sham methods as implemented in the CRYSTAL code. The effect of basis sets of increasing size on ϵ and χ(2) is explored. Five different levels of theory, namely, local-density approximation, generalized gradient approximation (PBE), hybrids (B3LYP and PBE0), and HF are compared using the experimental and theoretical structures corresponding not only to the tetragonal geometry Imathd2 at room temperature but also to the orthorhombic phase Fdd2 at low temperature. Comparison between the two phases and their optical behavior is made. The calculated results for the tetragonal phase are in good agreement with the experimental data.