The Dipole Moment Surface and the Vibrational Transition Moments of H2O

Abstract

We present here a detailed calculation of the vibrational transitions of the water molecule in its electronic ground state. The results have been obtained through approximately 5000 hr of computing (CPU) time and constitute the most complete and accurate calculation that exists for water. We locate 412 vibrational energy levels below 30 000 cm−1 and compute the transition moments for all of the 80 000 transitions between these states. More than 99% of the partition function is represented by our computed energy levels for temperatures up to 5000 K. For the fundamental bands the agreement of our results with the experimental values is as good as, or better than, the most accurate theoretical results previously published. The present work is sufficiently accurate to allow the calculation of weak band intensities directly from the theoretical dipole moment surface. Previous computations have been limited to the intensities of only a few overtone and combination bands. Our calculation gives better agreement with experiment than the best of these investigations, and it includes a complete set of many more bands. The dipole moment surface is computed by use of the CASSCF-CI theory, and the vibrational energies and the vibrationally averaged transition moments are determined through the Morse oscillator rigid bender internal dynamics approach. Our contracted GIGTO basis set for the CASSCF calculation has been chosen by first constructing a large primitive set of 132 GTOs of s, p, d, and f orbitals on oxygen and s, p, and d orbitals on hydrogen, and then finding the contracted set that produced the best agreement with the results obtained with the uncontracted set for nuclear geometries involving large displacements of the bending and stretching coordinates. The principal aim of the present study is to obtain a detailed knowledge of the monochromatic opacity of water in various types of astrophysical objects (red giants, brown dwarfs, cool dwarf stars, proto-stellar clouds, the outer edge of accretion disks, the interstellar medium, etc). but since gaseous water plays an important role in many other contexts we believe that our results can also be used, for example, in high-temperature technology or in studies of the Earth′s atmosphere where a complete and accurate knowledge of the water absorption is necessary.