.. _adsorb_add:

*******************
Adding an Adsorbate
*******************

The next step in this tutorial will go through how to add an adsorbate to a 
surface. Here we will consider the adsorption of CO\ :sub:`2`. To work out 
the energy of adsorption (E\ :sub:`ads`), we need to first know the energy 
of the surface (MgO), the energy of the adsorbate (CO\ :sub:`2`) and the 
energy of the combined system (MgO--CO\ :sub:`2`):

.. math::
   E_{ads} = E_{MgO_{suf}+CO_2} - E_{MgO_{surf}} - E_{CO_2}

We have already calculated the optimised energy of MgO in the previous examples. 
We now need to do the same for both CO\ :sub:`2` and MgO--CO\ :sub:`2`.

`1. Optimised Energy of Adsorbate`_
=======================================

.. note:: The scripts for this step of the tutorial can be found in 
   ``mgo_surface/4_co2_opt/``.

Unlike in the previous example, there is no need to consider **MM** 
methods for CO\ :sub:`2` as it will be part of the QM region when the 
adsorbed system is calculated. 
Instead the script ``co2_opt_energy.py`` contains a simple QM-only 
calculation:

.. literalinclude:: ../../samples/mgo_surface/4_co2_opt/co2_opt_energy.py
    :lines: 6-21

For consistency, B3LYP density functional calculations are used as 
in the MgO calculation, and the basis set used for CO\ :sub:`2`
is the same Ahlrichs pVDZ basis used for oxygen in MgO.

`2. Optimised Energy of the Adsorbed State`_
================================================

.. note:: The scripts for this step of the tutorial can be found in 
   ``mgo_surface/5_mgo+co2_opt/``.

The combined MgO--CO\ :sub:`2` system requires a full QM/MM calculation
of a similar nature to the bare MgO surface.

In the script ``mgoco2.py`` the combined system is first created by 
appending a CO2 molecule to the optimised MgO cluster. This is the
starting structure for the optimisation and is saved to a new 
punch file ``mgoco2start.pun``.

.. literalinclude:: ../../samples/mgo_surface/5_mgo+co2_opt/mgoco2.py
    :lines: 8-12

We now have a total of 2373 atoms in the system (with indices 0-2372). 
The last three of these are the atoms of the adsorbed CO\ :sub:`2` molecule.
The ``qm_region`` now contains both *region 1* of the MgO cluster and 
CO\ :sub:`2`: 

.. literalinclude:: ../../samples/mgo_surface/5_mgo+co2_opt/mgoco2.py
    :lines: 14-16

Similarly, the active optimisation region contains MgO regions 1-3 
plus CO\ :sub:`2`: 

.. literalinclude:: ../../samples/mgo_surface/5_mgo+co2_opt/mgoco2.py
    :lines: 24-25

The QM/MM setup is otherwise very similar to the MgO optimisation
step, except that the basis ``mgoco2_nwchem.basis`` passed to the 
NWChem interface contains basis functions for both MgO and CO\ :sub:`2`.

With NWChem running on 4 cores, the geometry optimisation should complete
in approximately 90 minutes.

As in previous stages, the resulting optimised coordinates are saved to a new 
punch file ``mgoco2end.pun``, and the result is validated against
a pre-calculated optimised energy.

You can also visualise the optimised geometry of the combined system
using the file ``mgoco2end.xyz``. You should see that the CO\ :sub:`2`
has been bent away from the initial geometry on adsorption to MgO.

.. note:: Once all three optimised energies have been found, the 
   energy of adsorption can be calculated using the formula above.
   You should obtain a result of -0.0245 a.u. (-15.4 kcal/mol).