A first principle study of terahertz (THz) spectra of acephate

Using first principles calculations based on the density functional method we have studied THz spectra of molecular and solid forms of acephate. The computations are preformed using different functionals for both isolated-molecule as well as solid-state periodic system. The computed infrared spectra are compared with experimental data using FTIR techniques and terahertz time-domain spectroscopy (THz-TDS) over a wide range of frequencies method. While isolated-molecule calculations of the harmonic frequencies were found to be deficient to reproduce the experimental spectrum, the same calculations performed in the solid-state have achieved better agreement with observed THz spectral characters. Detailed normal mode analyses have been carried out for both molecular and solid-state calculations. Our results show that the normal modes at low frequencies not only correspond to the inter-molecular vibrations but also arise due to the coupling between inter- and intra-molecular motions, as well as due to the formation of the hydrogen bonds between molecules in the crystal. Moreover Car-Parrinello molecular dynamics is used to obtain dipole-active vibrations at room temperature, thus revealing non-negligible temperature effects due to anharmonicity.