Subject: Re: CCL:G98 TD/CIS Opposite sign of the components of length- a

From: "Jens Spanget-Larsen" <[email-masked]>
Date: Wed, 5 Jun 2002 11:59:52 +0100

Hello Peter:

> I have some questions regarding the output os CIS and TD jobs in G98
> (Rev. A7). I can't find anything on these issues in Gaussian
> documentation or past CCL posts, although the questions are rather
> simple. So I hope it makes sense...
> 
> The output of TD and CIS calculations give the x, y, and z
> components of the "Ground to excited state Transition electric
> dipole moments", "Ground to excited state transition velocity dipole
> Moments", and the "Ground to excited state transition magnetic
> dipole Moments". 
> 
> 1) Why are the components of the electric and velocity derived
> transition dipole moments oppositely signed? I thought that they
> should be identical in a complete basis. I would have thought that
> the sign ought to be the same. Which values should I trust? 

The dipole length and dipole velocity "transition moments" are not 
identical. But the dipole length and dipole velocity forms of the 
oscillator strength, f(L) and f(V), should lead to identical results 
for exact wavefunctions (a.u.):

  f(L) = (2/3) |<i|r|j>|^2 (Ej - Ei)

  f(V) = (2/3) |<i|p|j>|^2 / (Ej - Ei)

Here r is the position vector and p the momentum operator. For 
approximate wavefunctions, f(L) and f(V) are no longer identical, but 
for calculational models of high quality the two values should be close.

> 2) Is it possible to say which of the computed oscillator strengths
> (dipole-length or dipole-velocity derived) provide the best estimate
> of experimental absorption intensities?

One value is not necessarily better than the other, but usually the 
value derived by the dipole length expression is quoted. But as long 
as the two values are close, you shouldn't worry!

> 3) I suppose that the computed electric and magnetic dipole moments
> are origin dependent. But are the magnetic transition dipole moments
> derived in a manner which make them related in to the transition
> velocity dipole moments, rather than the transition electric dipole
> moments? (e.g. in terms of origin dependence etc.)

The electric dipole moment should NOT be origin dependent, but the 
magnetic dipole moment is.

Yours, Jens >--<

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