# Part 2: Adding code

*See our* YouTube tutorial
*for a video demonstration of this section.*

The cookiecutter created a `<package_name>/`

directory where the MDAKit’s
code can live - now it’s time to start adding that code!

In this example, we are recreating the existing MDAnalysis RMSF analysis, so we will simply copy and paste this analysis class into a new file,

`rmsfkit/rmsfkit.py`

. The in-code documentation (which has been trimmed here for brevity) is written in reStructuredText syntax for building with Sphinx later.The contents of the file now resemble the following code block:

""" rmsfkit.py A package to perform RMSF analysis on molecular dynamics data. All code and documentation, both of which are trimmed for the sake of brevity, are taken from the MDAnalysis Python package. Used under GPLv2+. """ from MDAnalysis.analysis.base import AnalysisBase import numpy as np class RMSF(AnalysisBase): r"""Calculate RMSF of given atoms across a trajectory. Note ---- No RMSD-superposition is performed; it is assumed that the user is providing a trajectory where the protein of interest has been structurally aligned to a reference structure. The protein also has to be whole because periodic boundaries are not taken into account. Run the analysis with :meth:`RMSF.run`, which stores the results in the array :attr:`RMSF.results.rmsf`. """ def __init__(self, atomgroup, **kwargs): r"""Parameters ---------- atomgroup : AtomGroup Atoms for which RMSF is calculated verbose : bool (optional) Show detailed progress of the calculation if set to ``True``; the default is ``False``. Raises ------ ValueError raised if negative values are calculated, which indicates that a numerical overflow or underflow occurred Notes ----- The root mean square fluctuation of an atom :math:`i` is computed as the time average .. math:: \rho_i = \sqrt{\left\langle (\mathbf{x}_i - \langle\mathbf{x}_i\rangle)^2 \right\rangle} No mass weighting is performed. This method implements an algorithm for computing sums of squares while avoiding overflows and underflows :cite:p:`Welford1962`. References ---------- .. bibliography:: :filter: False Welford1962 """ super(RMSF, self).__init__(atomgroup.universe.trajectory, **kwargs) self.atomgroup = atomgroup def _prepare(self): self.sumsquares = np.zeros((self.atomgroup.n_atoms, 3)) self.mean = self.sumsquares.copy() def _single_frame(self): k = self._frame_index self.sumsquares += (k / (k+1.0)) * (self.atomgroup.positions - self.mean) ** 2 self.mean = (k * self.mean + self.atomgroup.positions) / (k + 1) def _conclude(self): k = self._frame_index self.results.rmsf = np.sqrt(self.sumsquares.sum(axis=1) / (k + 1)) if not (self.results.rmsf >= 0).all(): raise ValueError("Some RMSF values negative; overflow " + "or underflow occurred")

We have now added the

`RMSF`

analysis class to our package. Finally, to make this class easier to access, we import it in`__init__.py`

by adding:from .rmsfkit import RMSF

## Progress: MDAKit requirements

**✓ Uses MDAnalysis**- the added code uses the MDAnalysis`AnalysisBase`

class, and takes MDAnalysis`AtomGroup`

objects as inputs.**✓ Open source + OSI license***Versioned + on a version-controlled repository***✓ Designated authors and maintainers***(At least) minimal documentation**(At least) minimal regression tests***✓ Installable as a standard package****✓ (Recommended) community information available***(Recommended) on a package distribution platform*