TREXIO Tutorial

This tutorial covers some basic use cases of the TREXIO library based on the Python API. At this point, it is assumed that the TREXIO Python package has been successfully installed on the user machine or in the virtual environment. If this is not the case, feel free to follow the installation guide.

Importing TREXIO

First of all, let's import the TREXIO package.

If no error occurs, then it means that the TREXIO package has been successfully imported. Within the current import, TREXIO attributes can be accessed using the corresponding trexio.attribute notation. If you prefer to bound a shorter name to the imported module (as commonly done by the NumPy users with import numpy as np), this is also possible. To do so, replace import trexio with import trexio as tr for example. To learn more about importing modules, see the corresponding page of the Python documentation.

Creating a new TREXIO file

TREXIO currently supports two back ends for file I/O:

  1. TREXIO_HDF5, which relies on extensive use of the HDF5 library and the associated binary file format. This back end is optimized for high performance but it requires HDF5 to be installed on the user machine.

  2. TREXIO_TEXT, which relies on basic I/O operations that are available in the standard C library. This back end is not optimized for performance but it is supposed to work "out-of-the-box" since there are no external dependencies.

Armed with these new definitions, let's proceed with the tutorial. The first task is to create a TREXIO file called benzene_demo.h5. But first we have to remove the file if it exists in the current directory

We are now ready to create a new TREXIO file:

This creates an instance of the trexio.File class, which we refer to as demo_file in this tutorial. You can check that the corresponding file called benzene_demo.h5 exists in the root directory. It is now open for writing as indicated by the user-supplied argument mode='w'. The file has been initiated using TREXIO_HDF5 back end and will be accessed accordingly from now on. The information about back end is stored internally by TREXIO, which means that there is no need to specify it every time the I/O operation is performed. If the file named benzene_demo.h5 already exists, then it is re-opened for writing (and not truncated to prevent data loss).

Writing data in the TREXIO file

Prior to any work with TREXIO library, we highly recommend users to read about TREXIO internal configuration, which explains the structure of the wavefunction file. The reason is that TREXIO API has a naming convention, which is based on the groups and variables names that are pre-defined by the developers. In this Tutorial, we will only cover contents of the nucleus group. Note that custom groups and variables can be added to the TREXIO API.

In this tutorial, we consider benzene molecule (C6H6) as an example. Since benzene has 12 atoms, let's specify it in the previously created demo_file. In order to do so, one has to call trexio.write_nucleus_num function, which accepts an instance of the trexio.File class as a first argument and an int value corresponding to the number of nuclei as a second argument.

In fact, all API functions that contain write_ prefix can be used in a similar way. Variables that contain _num suffix (or dim type) are important part of the TREXIO file because some of them define dimensions of arrays. For example, nucleus_num variable corresponds to the number of atoms, which will be internally used to write/read the nucleus_coord array of nuclear coordinates. In order for TREXIO files to be self-consistent, the data in the file cannot be overwritten.

The number of atoms is not sufficient to define a molecule. Let's first create a list of nuclear charges, which correspond to benzene.

According to the TREX configuration file, there is a charge attribute of the nucleus group, which has float type and [nucleus_num] dimension. The charges list defined above fits nicely in the description and can be written as follows

Note: TREXIO function names only contain parts in singular form. This means that, both write_nucleus_charges and write_nuclear_charges are invalid API calls. These functions simply do not exist in the trexio Python package and the corresponding error message should appear.

Alternatively, one can provide a list of nuclear labels (chemical elements from the periodic table) that correspond to the aforementioned charges. There is a label attribute of the nucleus group, which has str type and [nucleus_num] dimension. Let's create a list of 12 strings, which correspond to 6 carbon and 6 hydrogen atoms:

This can now be written using the corresponding trexio.write_nucleus_label function:

Two examples above demonstrate how to write arrays of numbers or strings in the file. TREXIO also supports I/O operations on single numerical or string attributes. In fact, in this Tutorial you have already written one numerical attribute: nucleus_num. Let's now write a string 'D6h', which indicates a point group of benzene molecule. According to the TREX configuration file, point_group is a str attribute of the nucleus group, thus it can be written in the demo_file as follows

Writing NumPy arrays (float or int types)

The aforementioned examples cover the majority of the currently implemented functionality related to writing data in the file. It is worth mentioning that I/O of numerical arrays in TREXIO Python API relies on extensive use of the NumPy package. This will be discussed in more details in the section about reading data. However, TREXIO write_ functions that work with numerical arrays also accept numpy.ndarray objects. For example, consider a coords list of nuclear coordinates that correspond to benzene molecule

Let's take advantage of using NumPy arrays with fixed precision for floating point numbers. But first, try to import the numpy package

You can now convert the previously defined coords list into a numpy array with fixed float64 type as follows

TREXIO functions that write numerical arrays accept both lists and numpy arrays as a second argument. That is, both trexio.write_nucleus_coord(demo_file, coords) and trexio.write_nucleus_coord(demo_file, coords_np) are valid API calls. Let's use the latter and see if it works

Congratulations, you have just completed the nucleus section of the TREXIO file for benzene molecule! Note that TREXIO API is rather permissive and do not impose any strict ordering on the I/O operations. The only requirement is that dimensioning (see dim type) attributes have to be written in the file before writing arrays that depend on these variables. For example, attempting to write nucleus_charge or nucleus_coord fails if nucleus_num has not been written.

TREXIO error handling

TREXIO Python API provides the trexio.Error class which simplifies exception handling in the Python scripts. This class wraps up TREXIO return codes and propagates them all the way from the C back end to the Python front end. Let's try to write a negative number of basis set shells basis_num in the TREXIO file.

The error message says Invalid (negative or 0) dimension, which indicates that the user-provided value -256 is not valid.

As mentioned before, the data in the TREXIO file cannot be overwritten. But what happens if you accidentally attempt to do so? Let's have a look at the write_nucleus_num function as an example:

The API rightfully complains that the target attribute already exists and cannot be overwritten.

Alternatively, the aforementioned case can be handled using trexio.has_nucleus_num function as follows

TREXIO functions with has_ prefix return True if the corresponding variable exists and False otherwise.

What about writing arrays? Let's try to write an list of 48 nuclear indices instead of 12

According to the TREX configuration file, nucleus_index attribute of a basis group is supposed to have [nucleus_num] elements. In the example above, we have tried to write 4 times more elements, which might lead to memory and/or file corruption. Luckily, TREXIO internally checks the array dimensions and returns an error in case of inconsistency.

Closing the TREXIO file

It is good practice to close the TREXIO file at the end of the session. In fact, trexio.File class has a destructor, which normally takes care of that. However, if you intend to re-open the TREXIO file, it has to be closed explicitly before. This can be done using the close method, i.e.

Good! You are now ready to inspect the contents of the benzene_demo.h5 file using the reading functionality of TREXIO.

Reading data from the TREXIO file

First, let's try to open an existing TREXIO file in read-only mode. This can be done by creating a new instance of the trexio.File class but this time with mode='r' argument. Back end has to be specified as well.

When reading data from the TREXIO file, the only required argument is a previously created instance of trexio.File class. In our case, it is demo_file_r. TREXIO functions with read_ prefix return the desired variable as an output. For example, nucleus_num value can be read from the file as follows

The function call assigns nucleus_num_r to 12, which is consistent with the number of atoms in benzene that we wrote in the previous section.

All calls to functions that read data can be done in a very similar way. The key point here is a function name, which in turn defines the output format. Hopefully by now you got used to the TREXIO naming convention and the contents of the nucleus group. Which function would you call to read a point_group attribute of the nucleus group? What type does it return? See the answer below:

The trexio.read_nucleus_point_group function call returns a string D6h, which is exactly what we provided in the previous section. Now, let's read nuclear charges and labels.

The values are consistent with each other and with the previously written data. Not bad. What about the format of the output?

This makes sense, isn't it? We have written a list of nuclear labels and have received back a list of values from the file. What about nuclear charges?

Looks like trexio.read_nucleus_charge function returns a numpy.ndarray even though we have provided a python-ic list to trexio.write_nucleus_charge in the previous section. Why is it so? As has been mentioned before, TREXIO Python API internally relies on the use of the NumPy package to communicate arrays of float-like or int-like values. This prevents some memory leaks and grants additional flexibility to the API. What kind of flexibility? Check this out:

It means that the default precision of the TREXIO output is double (np.float64) for arrays of floating point numbers like nucleus_charge. But what if you do not need this extra precision and would like to read nuclear charges in single (np.float32) or even reduced (e.g. np.float16) precision? TREXIO Python API provides an additional (optional) argument for this. This argument is called dtype and accepts one of the NumPy data types. For example,

Reading multidimensional arrays

So far, we have only read flat 1D arrays. However, we have also written a 2D array of nuclear coordinates. Let's now read it back from the file:

We can see that TREXIO returns a 2D array with 12 rows and 3 columns, which is consistent with the nucleus_coord dimensions [nucleus_num, 3]. What this means is that by default TREXIO reshapes the output flat array into a multidimensional one whenever applicable. This is done based on the shape specified in the TREX configuration file.

In some cases, it might be a good idea to explicitly check that the data exists in the file before reading it. This can be achieved using has_-suffixed functions of the API. For example,


In this Tutorial, you have created a TREXIO file using HDF5 back end and have written the number of atoms, point group, nuclear charges, labels and coordinates, which correspond to benzene molecule. You have also learned how to read this data back from the TREXIO file and how to handle some TREXIO errors.