The dipole moment of a water molecule can change when an external electric field is applied, as predicted by ab initio calculations. Ab initio calculations are computational methods used in quantum chemistry to solve the Schrödinger equation for molecular systems from first principles, without any empirical data.When an external electric field is applied to a water molecule, the electric field interacts with the molecule's electron distribution, causing it to polarize. This means that the electron cloud around the oxygen atom is shifted towards the positive end of the electric field, while the electron cloud around the hydrogen atoms is shifted towards the negative end of the electric field.As a result, the dipole moment of the water molecule increases in the direction of the applied electric field. The magnitude of this increase depends on the strength of the electric field and the polarizability of the water molecule, which is determined by the molecule's electronic structure.Ab initio calculations can be used to predict the change in the dipole moment of a water molecule in response to an external electric field. These calculations involve solving the Schrödinger equation for the water molecule in the presence of the electric field, taking into account the effects of the field on the molecule's electronic structure and the resulting changes in the dipole moment.In summary, the dipole moment of a water molecule changes with the addition of an external electric field as predicted by ab initio calculations. The dipole moment increases in the direction of the applied electric field, with the magnitude of the increase depending on the strength of the field and the polarizability of the water molecule.