Potential energy and dipole moment surfaces of HCO⁻ for the search of H⁻ in the interstellar medium

Potential energy and permanent dipole moment surfaces of the electronic ground state of formyl negative ion HCO⁻ are determined for a large number of geometries using the coupled-cluster theory with single and double and perturbative treatment of triple excitations ab initio method with a large basis set. The obtained data are used to construct interpolated surfaces, which are extended analytically to the region of large separations between CO and H⁻ with the multipole expansion approach. We have calculated the energy of the lowest rovibrational levels of HCO⁻ that should guide the spectroscopic characterization of HCO⁻ in laboratory experiments. The study can also help to detect HCO⁻ in the cold and dense regions of the interstellar medium where the anion could be formed through the association of abundant CO with still unobserved H⁻.