Exact mid-IR quantum vibrational spectra of neutral water clusters

We use selected configuration interaction to calculate the zero-temperature mid-infrared (2800-3800 cm$^{-1}$) vibrational spectra of a water monomer, dimer, trimer, and hexamer in its cage and prism geometries. We use the recently introduced, accurate q-AQUA-pol potential energy surface along with the n-mode representation of the potential to facilitate grid-based quadrature and integral storage. Within selected configuration interaction, we introduce a new approach to the calculation of spectra that is complementary to eigenstate enumeration. In the new approach, we calculate the spectrum using the response-vector method, and the system of linear equations is solved using a basis of configurations that are optimally selected at each frequency of interest. We compare our spectra to previous studies, and highlight limitations of the local monomer approximation. To the best of our knowledge, our hexamer spectra are the most accurate ones reported to date.