Photonics
---------

Fluorescent and TADF (1\ :sup:`st` and 3\ :sup:`rd` generation) emitters 
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In this example we will characterise some of the core photophysical properties of a prototypical thermally-activated delayed fluorescence emitter,
including the emission energy (and colour), singlet-triplet splitting energy, spin-orbit coupling, and a visual representation of the excited states (NTOs).


We will be using DMAC_TRZ\ :cite:p:`DMAC-TRZ` as our example molecule:

.. image:: /_static/examples/DMAC_TRZ/DMAC_TRZ.png
  :alt: DMAC_TRZ

You can download the input file :download:`here </_static/examples/DMAC_TRZ/DMAC_TRZ.cml>`.

Submit
======

As with all calculations starting from a hand-drawn structure, we begin by optimising the molecule's geometry by selecting a calculation from the
`Optimisation` menu:

.. image:: /_static/examples/DMAC_TRZ/Opt.png

For this example we will be using the popular PBE0 functional, but most hybrid DFT functionals will provide reasonable results. Other popular
choices included B3LYP and M062X.

If you are looking to predict the properties of the emitter in a particular solvent, that can now be chosen from the subsequent list (or select `Gas Phase`
for a solvent agnostic calculation):

.. image:: /_static/examples/DMAC_TRZ/Solvent.png

Finally, we must make a choice of basis set. For smaller emitters, or when more computational time is available, a triple-zeta basis set offers a more
reliable result, but for larger molecules basis sets of double-zeta quality are normally sufficient for at least qualitative results. For this example,
we will choose the popular 6-31G\*\* set:

.. image:: /_static/examples/DMAC_TRZ/Basis.png

For the calculation of the excited states themselves, we will be utilising time-dependent DFT theory (optionally with the Tamm Dancoff approximation (TDA)).
This can be selected from the `Excited states` menu:

.. image:: /_static/examples/DMAC_TRZ/TDA.png

For a TADF emitter, the relative position of the lowest energy singlet and triplet states is a crucial parameter for evaluating the degree of reverse intersystem-crossing (RISC),
so we will be calculating both states simultaneously. For a pure fluorescent emitter, the triplet states could be skipped if they are not of interest. We will then
select the same options for the functional, solvent, and basis set as we did for the ground state optimisation:

.. image:: /_static/examples/DMAC_TRZ/TDA-basis.png

Finally, we will also calculate the geometry of the first singlet excited state from the `Optimised S(1)` menu. This will allow us to calculate a more accurate value for the emission energy:

.. image:: /_static/examples/DMAC_TRZ/Opt-S1.png

To recap, we have now queued up 3 calculations in series, which are:

  #. Optimisation of the ground state geometry
  #. Calculation of the first 10 singlet and triplet excited states
  #. Optimisation of the S(1) geometry
 
Which we can now submit for processing.

Analysis
========

This documentation is coming soon!


Phosphorescent (2\ :sup:`nd` generation) emitters
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The process to characterise a phosphorescent emitter (also known as a 2\ :sup:`nd` generation emitter) is largely the same as for (thermally activated delayed) fluorescence emitters.
The main differences are:

 #. The emissive state is a triplet, rather than a singlet. This enables us to optimise the orbitals of the triplet state directly with relative ease, which is not usually the case
    for the excited singlet state (because we'll normally end up with the singlet ground state instead).
 #. Most phosphorescent emitters are organometallics, containing one or more heavy metal atoms in their core. This necessitates a more careful choice of basis set.
 
In this example, we will characterise some of the key properties of the well-documented phosphorescent emitter, Ir(ppy)\ :sub:`3`\ :cite:p:`Ir(ppy)3`:

.. image:: /_static/examples/Ir(ppy)3/Ir(ppy)3.png
  :alt: Ir(ppy)3
  
You can download the input file :download:`here </_static/examples/Ir(ppy)3/Ir(ppy)3.cml>`.

Submit
======

We will begin in much the same way as before, with an initial geometry optimisation calculation to find a stationary point for the molecular geometry. Both PBE0 and especially 
B3LYP are very popular in this field, and either is likely to give sensible results. For this demonstration, we will be using B3LYP:

.. image:: /_static/examples/Ir(ppy)3/Opt.png

Because of the iridium atom at the centre of the molecule, we cannot use the same 6-31G\*\* basis set as before (as this typically only includes atoms up to Kr). We could choose
a more modern basis set from the Karlsruhe family (such as def2-TZVP), but for this example we will instead use a mixed basis set,\ :cite:p:`LANL,SBKJC` using 6-31+G\*\* for the light atoms and SBKJC-VDZ
for iridium (with an additional effective core potential for the metal). This option can be selected from the `External Basis Sets` option:

.. image:: /_static/examples/Ir(ppy)3/Basis.png

After the optimisation, we will queue a normal excited states calculation with TD-DFT (here with the Tamn-Dancoff approximation). This calculation will yield a good approximation
for the lowest energy singlet and triplet states:

.. image:: /_static/examples/Ir(ppy)3/TDA.png

Next, we will optimise the structure of the emissive triplet state (T\ :sub:`1`). Here, we could chose to again use TD-DFT as we did for the TADF emitter above,
but instead we will use standard DFT and manually set the multiplicity of the molecule to 3. Because there is no triplet ground state accessible for this molecule 
(because it has a closed shell), this will naturally converge to the first triplet excited state instead, allowing us to directly optimise the orbitals of the excited state.
This type of calculation is often referred to as ΔSCF or ΔDFT, and can be selected from the `Unrestricted Triplet` menu:

.. image:: /_static/examples/Ir(ppy)3/Opt_state.png

Finally, we will queue a single point calculation using a singlet configuration. This calculation will use the geometry of the excited triplet state, but will relax the orbitals back
to the ground state. The difference in energy between these two configurations will give us the phosphorescence emission energy:

.. image:: /_static/examples/Ir(ppy)3/SP.png

We now have a total of 4 calculations queued, which we can submit:

.. image:: /_static/examples/Ir(ppy)3/Submit.png

Analysis
========

This documentation is coming soon!