First-principles infrared spectrum of nitric acid and nitric acid monohydrate crystals

The spectra of nitric acid and nitric acid monohydrate crystals, including both vibration frequencies and infrared intensities, have been calculated from first principles using the methodology developed for the SIESTA program. In this method, infrared intensities are evaluated from the macroscopic polarization changes with atomic displacements per normal mode. The process requires the calculation of Born charges associated with atomic displacements. The actual value of the Born tensor elements gives interesting information on the electronic nature of the bonds. The application of this method to the two systems of atmospheric relevance studied here is particularly interesting. The calculated spectra are compared to previous experimental transmission spectra of these species with overall satisfactory agreement. The calculations provide a useful tool for the interpretation and assignment of the observed spectra.