2.6. ReaxFF: Adsorption on ZnS(110)¶
This tutorial illustrates ReaxFF parametrization for ZnS and H₂S on ZnS(110).
The tutorial teaches how to
- Combine several different force fields from the AMS ReaxFF library into a good initial guess
- Select training jobs to enter the training set
The input files have already been prepared. If you simply want to run the
parametrization without preparing the input, copy the contents of
$AMSHOME/scripting/scm/params/examples/ZnS_ReaxFF to a new empty
To open the example in the graphical user interface (GUI): SCM → ParAMS,
File → Open the
job_collection.yaml file in the new directory.
You can then run the parametrization.
2.6.1. Mix force fields from the AMS ReaxFF library¶
The ReaxFF force fields included with AMS cover a large part of the periodic table, however, far from all interactions are included.
In this tutorial, the goal is to model H₂S adsorption on a ZnS surface. No ZnS ReaxFF force field is included with AMS, but the following force fields might help:
|LiS.ff||Sulfur||Islam et al.|
|Mue2016.ff||Organic S-containing compounds||Müller and Hartke|
|ZnOH.ff||Zn and ZnO surfaces||Raymand et al.|
A ReaxFF force field contains parameters in the following categories: general (GEN), atomic (ATM), bond (BND), angle (ANG), off-diagonal (OFD), torsional (TOR), hydrogen bond (HBD).
Here, we’ll use the following initial parameters from the above force fields:
- LiS.ff: ATM:S, BND:S.S, ANG:S.S.S
- Mue2016.ff: ATM:H, BND:S.H, ANG:H.S.H
- ZnOH.ff: GEN, ATM:Zn, BND:H.H, BND:Zn.H, BND:Zn.Zn, BND:Zn.O*, ANG:Zn.Zn.O*, ANG:Zn.O*.Zn, ANG:Zn.O*.O*, ANG:O*.Zn.O*, HBD:O*.H.O*
For ZnOH.ff, the asterisks in e.g. BND:Zn.O* indicate that we will instead copy those parameters to initialize the BND:Zn.S block. That is, the Zn-O interaction in ZnOH.ff will be used as first guess for the Zn-S interaction in the new force field.
2.6.2. Choose the parameters to optimize¶
We will not optimize (deactivate)
- Any GEN (general) parameters
- Any ATM (atomic) parameters
- Any parameters with category
DoNotOptimize(these should never be optimized)
- Any BND (bond) parameters for H-Zn or H-H (there is no training data for those interactions)
- Any BND (bond) parameters for π-bonds (there is no training data)
- Any ANG (angle) parameters for S-S-S (there is no training data)
- Any parameters with value 0 (usually means “off”)
We will optimize (activate):
|BND||S-S, H-S, Zn-Zn, S-Zn||
|ANG||H-S-H, S-Zn-Zn, Zn-S-Zn, S-S-Zn, S-Zn-S||
2.6.3. ZnS training and validation set¶
The training data contains
- Energy-volume scans of the zinc blende and rock salt polymorphs of ZnS (wurtzite energy-volume scan in the validation set). See the GFN1-xTB tutorial for how to set up an energy-volume scan reference calculation in AMS and import it into ParAMS.
- Enthalpy of formation of the zinc blende, wurtzite, and rocksalt polymorphs of ZnS
- Optimized S-H bond length in H₂S, and H-S-H angle. See the GFN1-xTB tutorial or Import training data (GUI) tutorial for how to import the bond length from a geometry optimization job.
- Bond scan for the S-H bond and angle scan for the H-S-H angle in gaseous H₂S. See the ReaxFF: Gaseous H₂O tutorial for how to set up a bond scan reference calculation in AMS.
Clean ZnS(110) surface
- Optimized bond lengths and angles at a clean ZnS(110) surface. See also: Building Crystals and Slabs and Crystals and Surfaces
- Formation energy (surface energy) of the ZnS(110) surface. See also: Surface energy
- Forces on distorted ZnS(110) surfaces (*)
H₂S adsorbed on ZnS(110)
- Optimized Zn-S and H…S bond lengths for H₂S/ZnS(110)
- Adsorption energy of H₂S/ZnS(110). See also: Adsorption
- Atomic charges for a variety of systems
(*): The distorted ZnS(110) surfaces were obtained by geometry-optimizing the clean ZnS(110) surface using a previous iteration of the force field.
This tutorial does not cover how to set up all the reference calculations. In the GUI, if you switch to the Jobs panel and double-click in the Detail column, you can use the button Edit in AMSinput to see how the reference calculation was set up (approximately, some details like the maximum number of iterations for the geometry optimization may differ).
If you have run a reference calculation, in the ParAMS GUI you can choose Jobs → Add Job, and browse to the ams.rkf file from the job. That will let you import many properties (energy, forces, stress tensor, charges, …).
2.6.4. Run the ZnS ReaxFF parametrization¶
2.6.5. ZnS parametrization results¶
These results only give an example of a possible result. If you rerun the parametrization, your results will likely differ and could either be better or worse.
126.96.36.199. ZnS energies¶
|Reference (eV)||Prediction (eV)|
|ZnS enthalpy of formation (zincblende)||-2.737||-2.737|
|ZnS enthalpy of formation (rock salt)||-2.329||-2.360|
|H₂S adsorption energy on ZnS(110) (a)||-0.742||-0.559|
|ZnS(110) slab energy relative to bulk (b)||+0.124||+0.257|
(a): This adsorption energy was not calculated using the in-cell approach
(b): This energy can be converted into a surface energy