How to access OCP data?

[michelegalasso]

PyBaMM Version

23.5

Python Version

3.10.12

Describe the bug

I am trying to use the OCP curve of graphite as implemented in the "Chen2020_composite" parameter dataset, but when I try to access the data I get a TypeError. OCP curves of silicon and NMC implemented in the same dataset work as expected.

Steps to Reproduce

import pybamm

# load parameter values
parameter_values = pybamm.ParameterValues('Chen2020_composite')

# extract the OCP curves
graphite_ocp_Enertech_Ai2020 = parameter_values['Primary: Negative electrode OCP [V]']

# try to access the data
ocp_value = graphite_ocp_Enertech_Ai2020(0.2)

Relevant log output

Traceback (most recent call last):
  File "/home/michele/Documents/learning/python/PyBaMM/main.py", line 15, in <module>
    ocp_value = graphite_ocp_Enertech_Ai2020(0.2)
  File "/home/michele/Documents/learning/python/PyBaMM/.venv/lib/python3.10/site-packages/pybamm/input/parameters/lithium_ion/Chen2020_composite.py", line 318, in graphite_ocp_Enertech_Ai2020
    return pybamm.Interpolant(x, y, sto, name=name, interpolator="cubic")
  File "/home/michele/Documents/learning/python/PyBaMM/.venv/lib/python3.10/site-packages/pybamm/expression_tree/interpolant.py", line 116, in __init__
    if len(x) != len(children):
TypeError: object of type 'float' has no len()

[brosaplanella]

Does this work?

ocp_value = graphite_ocp_Enertech_Ai2020([0.2])

[michelegalasso]

Yes, this returns an object <class 'pybamm.expression_tree.interpolant.Interpolant'>, with child equal to 0.2.
However, I am not sure what does all that mean and how to obtain a vector of OCP in volts corresponding to different lithiation states of graphite.

[valentinsulzer]

You need to do something like graphite_ocp_Enertech_AI2020(pybamm.Symbol(0.2)).evaluate() to get a single value or graphite_ocp_Enertech_AI2020(pybamm.linspace(0,1)).evaluate() to get a vector

[michelegalasso]

Now it works, thank you!! I had no idea that I needed to use pybamm.linspace(). Is that documented anywhere?

[brosaplanella]

The linspace is documented in the docs (https://docs.pybamm.org/en/latest/source/api/expression_tree/array.html#pybamm.linspace) but there is no example of this. Transferring to a discussion as it might be useful to other users.

[michelegalasso]

Thank you!