Simulation of emission spectra

Questions
#1

Hi! For most of my research I’ve been using ORCA to simulate emission spectra and it does an excellent job at it. But since the software has several numerical steps sometimes it takes too long, even on small systems and it is common to explode my execution time at the cluster.

My question is: can Q-Chem simulate accurate fluorescence/phosphorescence spectra using a less time-demanding algorithm? Could you share an example of this feature in action?

Thank you so much.

#2

The short answer is yes, but some more information is needed. What level of theory are you intending to use, and what is a typical molecular size? (Perhaps a sample ORCA input file might shed light on things.)

#3

Thanks for your reply, @jherbert.

The molecules are small to medium size, usually, from 27 to 50 atoms using DFT (I can’t afford higher computational levels). It is possible that I may work with lanthanides in the future but usually I’m doing only molecules with C,N,O,S.

At this point any example using Q-Chem could work for comparison purposes, even something as simple as aniline, so I could justify purchasing Q-Chem.

#4

What you want is TDDFT, which is described here: 7.3.1 Brief Introduction to TDDFT‣ 7.3 Time-Dependent Density Functional Theory (TDDFT) ‣ Chapter 7 Open-Shell and Excited-State Methods ‣ Q-Chem 5.4 User’s Manual

For emission, the rate-limiting step is excited-state geometry optimization, as you then compute vertical excitation energies (which is less costly than optimization) at the excited-state minimum. There are some TDDFT+PCM options also for describing solvent effects, see: 7.3.4 TDDFT + PCM for Excitation Energies and Excited-State Properties‣ 7.3 Time-Dependent Density Functional Theory (TDDFT) ‣ Chapter 7 Open-Shell and Excited-State Methods ‣ Q-Chem 5.4 User’s Manual

#5

Hi
I am using ORCA and I am currently working on borane molecule with 40 atoms, I couldn’t find way to calculate vertical emission energies, can you please help me with that.
Thank

#6

For a system that large, you probably want to use TDDFT, and for emission this means using a non-Aufbau reference state. After running a ground-state DFT calculation to obtain MOs, use a $occupied input section to specify a non-Aufbau guess to represent the excited state of interest (e.g., HOMO → LUMO excitation). You will need to use a convergence algorithm such as MOM, IMOM, STEP, or SGM to converge a non-Aufbau solution to the Kohn-Sham equations. That state can then be used as a reference for TDDFT, with negative excitation energies corresponding to emission.

Solution for a non-Aufbau reference is described in Section 3 of this chapter:
https://doi.org/10.1016/B978-0-323-91738-4.00005-1