n-pi and pi-pi* assignment

[brijesh]

Hi All,
Can any one please help me in assigning the excitations n to pi* or pi to pi* in ricc2 excited state calculations?

Thank you
Best
Brijesh

[Christof]

Hi Brijesh,

the easiest way is to look at the dominant configuration of the excited state and then visually inspect the MOs which take part in this excitation. Pi and Pi* orbitals are easily identified and the lone-pair type orbitals are typically perpendicular to the pi plane and have a significant contribution on the atom where your Lewis structure would predict a lone-pair. Additionally n-pi * excitations often feature a low oscillator strength, due to selection rules based on local symmetry. A typical example are for instance carbonyl compounds, which feature a very weak n-pi* excitations (in case of fomaldehyde it is only vibronically allowed) and a  rather strong pi-pi* excitation.
Best regards,
Christof

[brijesh]

Hi Christof,
Thanks for your mail. Could you please also comment on how to visualize MOs? Should I use tmolex or any other package? And which file contains the MOs information?

Thanks
Best Regards
Brijesh

[Christof]

Hi Brijesh,

well, there are several possibilities. I usually run tm2molden to obtain a molden.input file and then use molden to visualize them. I've no experience with tmolex, but I'm sure this will work also...

Best regards,
Christof

[brijesh]

Thank you very much.

[antti_karttunen]

Hi Brijesh,

Like Cristof said, visual inspection of the important MOs is probably the easiest way to analyze the excitations. I would also like to highlight the possibility of analyzing density differences between the ground state and the excited state. This is a very nice way of visualizing ho the electron density changes. Mikael Johansson has written a very good tutorial about Turbomole and excited states and this tutorial also contains a section on "Visualizing how the electron moves when excited with CC2": https://extras.csc.fi/chem/courses/tm-2010/CSC_course_140110.pdf

The Turbomole manual has even more information. This is a very elegant method since you get all the information in one figure and don't have to pick and visualize the MOs separately. Definitely this might not be the suitable way for all situations, but I've found it an indispensable way of analyzing electronic excitations when there's lots of data to look at (also, I'm lazy and hate drawing up MO figures of excitations with contributions coming from different 5 MOs  )

Antti

[brijesh]

Thanks. I am going to try it.

best regards
Brijesh