# 2. Getting Started¶

Important

The ParAMS toolkit is built on top of the PLAMS library. For users new to scripting in the Amsterdam Modeling Suite we recommend to first familiarize themselves with PLAMS. A good place to start are the PLAMS scripting tutorials.

## 2.1. Installation¶

No need to install, ParAMS is included in the amspython stack.

## 2.2. Unit Tests¶

1. cd \$AMSHOME/scripting/scm/params/tests
2. ./run_tests.sh

## 2.3. First Steps with ParAMS¶

At its core ParAMS provides a unified interface to physico-chemical properties that can be extracted from an AMS calculation multiple optimization algorithms that aim to improve the accuracy of those properties.

There are three main components to the package:

• The Job Collection, which represents a set of chemical systems stored as plams.Molecule objects alongside AMS settings specific to each system. The combination of system and settings clearly defines a computational job. An example for a job could be a geometry optimization of a methane molecule with the calculation of nuclear gradients.
• A Data Set, which holds the definition of all properties that are relevant for the parameter optimization. These could be gradients, energy and geometry of the aforementioned methane job. The Data Set is responsible for the extraction and comparison of all properties to their reference values, providing a loss value metric.
• A parameter interface, serving as a translation layer between a parametric AMS engine and a vectorized representation of the parameters that are relevant to the optimization. Any available parameter interface in ParAMS can be used for re-parameterization.

Components interact with each other as follows:

>>> print(jc) # jc is a JobCollection instance
---
ID: Water01
ReferenceEngineID: None
AMSInput: |
properties
End
system
Atoms
O      0.0000000000      0.0000000000      0.5937200000
H      0.0000000000     -0.7654400000     -0.0083600000
H      0.0000000000      0.7654400000     -0.0083600000
End
End
...

>>> print(ds) # ds is a DataSet instance
---
Expression: energy('Water01')
Weight: 1.0
ReferenceValue: -0.09210963293745549
---
Expression: forces('Water01')
Weight: 1.0
ReferenceValue: |
array([[-4.41310133e-11,  1.24966204e-10, -1.38738596e-05],
[-2.19091605e-11, -2.01034720e-05,  6.93698245e-06],
[-2.19091605e-11,  2.01036055e-05,  6.93698245e-06]])
...

>>> params = LennardJonesParams() # params is a ParameterInterface instance
>>> params.names # parameter names
['eps', 'rmin']
>>> print(params.x) # parameter values
[0.0003, 3.0]
>>> jobresults = jc.run(params) # calculate all jobs in jc with our parameters
>>> loss       = ds.evaluate(jobresults)
>>> print(loss) # How close are the results calculated with params to their reference values?
2082.088800689759

>>> params.x   = [0.0004, 4.] # change the parameter values ...
>>> jobresults = jc.run(params) # ... re-calculate ...
>>> loss       = ds.evaluate(jobresults) # ... and re-evaluate.
2066.4096600168773


Note how in the above code block we are manually assigning different parameter values to our params instance. This is the entry point to the fourth component in ParAMS - the optimizer. Piecing it all together is a top-level Optimization class, which is managing the parameter fitting process based on user-defined settings. A simplified overview of the ParAMS parameterization loop is depicted in the figure below.

For a hands-on dive into the workings of the package, we recommend checking out the Examples section (and trying them yourself). If at any point you stumble upon a class, whose functionality is not entirely clear, make sure to look it up in the respective Components subsection. Most of these provide additional examples on the component’s functionality on top of introducing the complete API.

Users that are keen on building their own ParAMS scripts can consult the Architecture Quick Reference for a rough guide on how different components interact with each other.

Feel free to contact us at support@scm.com in case you encounter difficulties, bugs or have questions and comments regarding the package.