The dipole moment of a water molecule calculated using ab initio methods can vary depending on the level of theory and basis set used in the calculations. However, a commonly reported value for the dipole moment of water using ab initio methods is approximately 1.85 Debye. This value is obtained using high-level quantum chemistry calculations, such as the coupled-cluster method with single, double, and perturbative triple excitations CCSD T and a large basis set. Keep in mind that the experimental value for the dipole moment of water is around 1.85 Debye as well, so the ab initio calculations provide a good agreement with the experimental data.