  Accurate knowledge of the rovibronic near-infrared and visible spectra of
vanadium monoxide (VO) is very important for studies of cool stellar and hot
planetary atmospheres. Here, the required ab initio dipole moment and
spin-orbit coupling curves for VO are produced. This data forms the basis of a
new VO line list considering 13 different electronic states and containing over
277 million transitions. Open shell transition, metal diatomics are challenging
species to model through ab initio quantum mechanics due to the large number of
low-lying electronic states, significant spin-orbit coupling and strong static
and dynamic electron correlation. Multi-reference configuration interaction
methodologies using orbitals from a complete active space self-consistent-field
(CASSCF) calculation are the standard technique for these systems. We use
different state-specific or minimal-state CASSCF orbitals for each electronic
state to maximise the calculation accuracy. The off-diagonal dipole moment
controls the intensity of electronic transitions. We test finite-field
off-diagonal dipole moments, but found that (1) the accuracy of the excitation
energies were not sufficient to allow accurate dipole moments to be evaluated
and (2) computer time requirements for perpendicular transitions were
prohibitive. The best off-diagonal dipole moments are calculated using
wavefunctions with different CASSCF orbitals.
