2.4. Import training data (GUI)¶

2.4.1. Choose preferred units¶

ParAMS supports several commonly used units, so that you can work with the units that are common in your field.

When you import a job (see below), the import dialog contains a choice of units.

2.4.2. Reference calculation #1: Geometry optimization of a water molecule¶

Open AMSinput and switch to the panel

Set Task → Geometry Optimization

Draw a water molecule. It is good if it is a bit distorted.

Save with the name water.ams

Run the job

2.4.3. Charges and forces¶

In the ParAMS GUI, you can now see

• On the Jobs panel is a job called water with Task SinglePoint.

• On the Training Set panel are two entries Charges and Forces for the job water.

• On the Engines panel is a new entry forcefield;<emptySettings>;. This contains the settings used for the reference calculation.

2.4.4. Bonds and angles¶

To train to geometry data like bonds or angles, you should set the Task (for new job) to Geometry Optimization when importing a job.

This adds the job water (or water_1 if you first added the charges and forces - job names need to be unique). It is marked in red because it is not yet used in any training set entry.

On the Jobs panel, click twice at the job name and change the name to water_geoopt.

Then, add one of the O-H bond lengths:

To add the H-O-H angle:

You can also add dihedral angles in a similar way (for molecules with at least 4 atoms).

2.4.5. PES scans: bond scan, angle scan, or volume scan¶

• Bond scan: See the ReaxFF (basic): H₂O bond scan tutorial

• Angle scan: in the same way as importing a bond scan

• Dihedral scan: in the same way as importing a bond scan

• Volume scan: in the same way as importing a bond scan. See also the GFN1-xTB: Lithium fluoride tutorial.

For the above type of PESScan reference calculations, there are two options for Use ResultsImporter in the import dialog:

• Add Single Job. This, together with Task (for new job) → PESScan, sets up an equivalent PESScan to be run during the parameter optimization. This is illustrated in the ReaxFF (basic): H₂O bond scan tutorial.

• Add PESScan Singlepoints. This, together with Task (for new job) → SinglePoint sets up several singlepoint calculations on the frames from the PESScan to be run during the parameter optimization. This is illustrated in the GFN1-xTB: Lithium fluoride tutorial.

The pes property for Add Single Job is equivalent to the relative_energies property for Add PESScan Singlepoints. They extract the relative energy of the different frames (by default, relative to the minimum energy in the scan). It is the fundamental property to extract from a PES Scan (bond scan, angle scan, volume scan).

With Add PESScan Singlepoints, you can also add the forces and stresstensor for each frame, but only if you enabled the Save results for all PES points option when running the PES Scan. See the GFN1-xTB: Lithium fluoride tutorial for an example.

With Add Single Job (and Task PESScan), many constrained geometry optimizations are performed for each set of parameters, which can be time-consuming. With Add PESScan Singlepoints, only single-point calculations are performed, which is usually faster.

With Add Single Job (and Task PESScan, pes extractor), the GUI will give useful plots of the energy vs. the scan coordinate for both the reference values and calculated values during the parametrization. For this reason, we recommend to use this option and not Add PESScan Singlepoints.

2.4.6. Energies¶

For Lennard-Jones, ReaxFF, and DFTB you typically only want to fit relative energies (reaction energies), not absolute energies. This is because the absolute energy levels are different for different engines.

As an example, let’s add the reaction energy for propene combustion: C₃H₆(g) + (9/2) O₂(g) → 3 CO₂(g) + 3 H₂O(g). First, run the reference calculations and import the jobs:

Then, add the reaction energy. You can either work out the stoichiometric coefficients yourself, or use the automatic Balance feature of ParAMS (see below).

This is the reaction energy calculated at the UFF (ForceField) level of theory - it is very far from reality! For a more realistic reference value, run the reference calculations with DFT (ADF, BAND, or Quantum ESPRESSO) instead.

For PES Scans, the pes and relative_energies properties extract relative energies from the PES scan.

2.4.7. Import a molecular dynamics trajectory¶

You can import frames from a Molecular Dynamics trajectory. Typically, running a molecular dynamics is a simple way of sampling some distorted structures, and can be used for fitting especially the forces but also relative energies.

There are two ways:

• Run the MD simulation directly with the reference method (often DFT, i.e. you would run ab initio MD).

• Run the MD simulation with a parametrized method, use the Replay task of the AMS Driver to recalculate some of the frames with DFT, and add only the DFT-calculated frames to ParAMS. In particular, because training sets often need to be expanded iteratively, the initial MD simulation could be run with your current force field, if you have already run a parametrization.

2.4.7.1. Reference calculation #2: MD simulation of liquid Ar¶

Open AMSinput, switch to the panel.

File → Import coordinates, select the file Ar32.xyz

Set Task to Molecular Dynamics

Click the next to the Task

Set Number of steps to 70.

Set Timestep to 5 fs.

Set Sample frequency to 10.

Set Initial temperature to 2000 K.

Click the next to Trajectory Options

Set Write engine gradients to Yes. The “engine gradients” are the negative of the atomic forces. If you do not check this option, the gradients (negative forces) will not be saved to the trajectory and cannot be imported to ParAMS.

Save with the job uff_md

Run the calculation. It should finish in a few seconds.

If a dialog asks you to update the coordinates, select No.

You can select the job in AMSjobs and select SCM → Movie to visualize the trajectory.

Note

The initial velocities are random. The trajectory (and the forces on the atoms) will be different when you run the calculation yourself compared to the screenshots in this tutorial.

2.4.7.2. MD with reference method¶

Here, we will use the previous calculation as an MD simulation with the reference method. Normally, you would have instead used DFT (on the or panels, or using Quantum ESPRESSO or VASP) for the MD simulation.

Select the finished uff_md job in AMSjobs.

Job → Add to ParAMS (Ctrl-T)

In the dialog, select Use ResultsImporter: Add Trajectory SinglePoints.

Set Steps from 3 to 7

Set Step size to 2. This will import frames 3, 5, and 7 from the trajectory.

Set Properties to relative_energies and forces. Note: you can select multiple entries in the drop-down list!

Click Import

This adds:

• Three Singlepoint jobs to the job collection: uff_md_frame003, uff_md_frame005, and uff_md_frame007, containing the structures of those frames.

• Two relative energy expressions: uff_md_frame003-uff_md_frame005 and uff_md_frame007-uff_md_frame005, with the corresponding reference values. In this case, frame 5 had the lowest energy so the relative energies are taken relative to that frame. In your case, frame 3 or 7 may be the lowest energy frame.

• Three forces expressions for the three frames. If you select a force entry and switch to the Info panel in the bottom table, you can see the individual force components.

• An Engine containing the settings for the reference engine (in this case UFF, which corresponds to an empty [default] Engine ForceField block).

2.4.7.3. MD with fast method followed by Replay with reference method¶

The task Replay performs a series of single-point calculcations on structures taken from an ams.rkf file, for example a molecular dynamics trajectory.

Here, we will use the GFN1-xTB (DFTB) method to recalculate some of the points from the molecular dynamics trajectory generated with UFF. If you do not have a DFTB license, you could instead run the Replay calculation with BAND, or even ForceField (UFF).

SCM → New Input

Switch to the panel (or use a different engine if you do not have a DFTB license)

Task → Replay

Click the arrow next to the Task

Click the icon next to Restart from and browse to the ams.rkf file from the previous job. It is located inside the folder uff_md.results.

Set Frames to 3 5 7

Go to the Properties → Gradients, Stress tensor panel.

Set Nulear gradients to Yes. This is needed to save the calculated forces so that they can be imported into ParAMS.

Save the job with the name Ar32.ams and run the job.

Wait for the job to finish.

If you get a dialog asking you to completely replace the system, select No.

When the calculation has finished, import it into ParAMS:

Select the job in AMSjobs

Job → Add to ParAMS (Ctrl-T)

In the dialog, select Use ResultsImporter: Add Trajectory SinglePoints.

Set Steps from 1 to 3

Set Step size to 1. This will import frames 1, 2, and 3 from the Replay job (corresponding to frames 3, 5, and 7 from the original trajectory).

Set Properties to relative_energies and forces. Note: you can select multiple entries in the drop-down list!

Click Import

This adds:

• Three Singlepoint jobs to the job collection: Ar32_frame001, Ar32_frame002, and Ar32_frame003, containing the structures of those frames.

• Two relative energy expressions: Ar32_frame001-Ar32_frame002 and Ar32_frame003-Ar32_frame002, with the corresponding reference values. In this case, frame 2 had the lowest energy so the relative energies are taken relative to that frame. In your case, frame 1 or 3 may be the lowest energy frame.

• Three forces expressions for the three frames. If you select a force entry and switch to the Info panel in the bottom table, you can see the individual force components.

• An Engine containing the settings for the reference engine (in this case GFN1-xTB, which corresponds to an empty [default] Engine DFTB block).

The above is exactly how the training data for the Getting Started: Lennard-Jones tutorial was created.

2.4.8. Import a structure and settings from AMSinput¶

You can import a structure and AMS driver settings (for example, geometry optimization settings) from AMSinput. This can be useful if you

• want to import an experimental structure

• want to import a structure optimized with a DFT code not supported by ParAMS (codes other than AMS, VASP, or Quantum ESPRESSO)

See the Import experimental formation enthalpy into ParAMS section of the GFN1-xTB tutorial for an example of how to do this.

2.4.9. Import VASP or Quantum ESPRESSO calculations¶

You can import results from VASP or Quantum ESPRESSO single points, geometry optimizations, and molecular dynamics simulations.

For VASP calculations run outside AMS, open the ParAMS GUI and select Jobs → Add Job, select the OUTCAR file from the file dialog. This will convert the VASP output to AMS format - it may take a while. Then, you will see the familiar results importer dialog, where you can choose either Add Single Job or Add Trajectory Singlepoints, and select properties including energy, forces, and stresstensor.

For Quantum ESPRESSO calculations, open the ParAMS GUI and select Jobs → Add Job, select the jobname.out file from the file dialog. This will convert the output to AMS format - it may take a while. Then, you will see the familiar results importer dialog, where you can choose either Add Single Job or Add Trajectory Singlepoints, and select properties including energy, forces, and stresstensor.

/path/to/pw.x < jobname.in > jobname.out