# Total Energy Comparison

````{admonition} Overview
:class: overview

Questions:
- Does my total energy agree with the values reported from NIST?

Objectives:
- Apply a cut-off and periodic boundaries to the energy calculation and compare it to values reported by NIST.
````

```{admonition} Prerequisites

Make sure you have the functions from the previous page in a Jupyter notebook.

```

Now that we have accounted for periodic boundaries and a cut-off, we can compare our results to those from NIST.

````{tab-set-code} 

```{code-block} python
nist_3cut = -4.3515E+03
nist_4cut = -4.4675E+03

config1_file = "../data/sample_config1.txt"

coords, box_length = read_xyz(config1_file)
calc_3cut = calculate_total_energy(coords, box_length, 3)
calc_4cut = calculate_total_energy(coords, box_length, 4)

print(f"Cut off 3: {calc_3cut}")
print(f"Cut off 4: {calc_4cut}")
```

````

The code block above uses a special syntax in the print statment called a formatted string.
In Python, formatted strings, denoted by an f prefix, allow us to embed expressions and variables directly into a string. 
By using curly braces {} to wrap the expressions, the values of those expressions are evaluated and inserted into the string when it is printed. 

Now that we have our calculated values, we can compare our results to those computed by NIST.


````{tab-set-code} 

```{code-block} python
assert math.isclose(calc_3cut, nist_3cut, rel_tol=0.02)
assert math.isclose(calc_4cut, nist_4cut, rel_tol=0.02)
```

````