# I Dipole moment component

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1. Mar 31, 2017

### Konte

Hi everybody,

I want to ask a question about the dipole moment vector $\vec{d}$, with components expressed in the molecule-fixed axis. By permuting identical nuclei that compose a rigid molecule, I know how $d_x$, $d_y$, $d_z$ change.
But when dealing with non-rigid molecule, I no longer know how to direct the molecule fixed- axis?

Thank you everybody.

Konte

2. Mar 31, 2017

### Jilang

Could you work with expectation values?

3. Mar 31, 2017

### Konte

How? What does you mean?

Konte.

4. Mar 31, 2017

### blue_leaf77

In molecular mechanics, what is meant by rigid is usually the assumption that the molecule only rotates in space without any vibration or at least vibrates with very small amplitude. In reality of course the molecule vibrates and hence the concept of rigid molecule no longer applies however for some low vibrational quantum numbers where the vibrational eigenfunction is more localized around the equilibrium position, you can take the equilibrium position to determine the axis.

5. Apr 2, 2017

### DrDu

Look out for "Sayvez conditions".

6. Apr 2, 2017