TDDFT Transition Dipole Reference

[robagl9695]

I want to calculate the angle between the transition electric dipole of the first electronic excitation of two molecules, A and B.

In order to do that, I've already performed TD-DFT calculations on both molecules, and extracted from the output the coordinates for both vectors. However, to calculate the angle, I need to know if the transition dipole vector coordinates are calculated and displayed with respect to the Cartesian coordinate center or with respect to the Center-of-Mass of each molecule.

Can someone please clarify this?

[chris.hol]

Hi, they are calculated with respect to the Cartesian coordinate center. 

[robagl9695]

Just to be completely and absolutely sure...

The set of coordinates that I input are the ones used in the calculations, so the molecule is never rotated or translated during the calculations?

[hherbol]

Are you doing a geometry optimization?  If you set symmetry to c1 and just are doing escf to get the TDMS, then the coordinates shouldn't shift.

Also, you're just comparing the TDMs correct?  The origin of the TDM doesn't matter, as it's just a vector of the change in state densities (naively), so you can just compare the two vectors themselves.

For example, the angle between the x and y axis is 90 degrees.  Similarly the angle between [10, 0, 0] and [0, 20, 0] is still 90 degrees.  Origin shouldn't matter.

You are correct to be concerned with the possibility of rotation though, but your frame of reference in for the TDM shouldn't change if you're not doing a geometry optimization and you leave symmetry off.