Compatibility Issues with LangevinVelocityIntegrator

[robinwuff]

I have used the LangevinVelocityIntegrator on a one-dimensional potential, and it outputs its trajectory positions (and velocities, I think) as an array containing two values for each step. While I don't know the implementation reasons or which one of them is the actual position, I have found this leads to issues down the line:

• Systems with the LangevinVelocityIntegrator as a sampler fail to work with oneD_simulation_analysis_plot()
• The LangevinVelocityIntegrator does not work with replicaExchangeEnvelopingDistributionSampling at least during my testing due to the exchange criterion failing to calculate anything using the two-value array.
• This one is more of an incovenience: I have not found a way to conveniently slice one of those positional values out of the trajectory (pandas.DataFrame containing these arrays as objects) without looping (but I am no python expert). The LangevinIntegrator in contrast just contains the positions as floats.
• The documentation gives no hint as to what these pairs of values signify and I have not dug far enough into the code to understand, whether one of these values might be a half-step or just the position in the second dimension (which I should not have in my 1D case).

[robinwuff]

I am not sure whether this deserves its own issue, but since I like swapping out samplers I have found three different ways positions were stored for the three samplers I tried (all of this while doing 1D simulations, I cannot comment on higher dimensions):

• LangevinIntegrator: Positions neatly stored as floats, very convenient for plotting and calculations.
• LangevinVelocityIntegrator: Positions stored as value pairs in arrays as described above.
• MetropolisMonteCarloInterator: Positions stored as single values, but each of them in an array containing only that one value. This makes for some surprises when switching from Langevin to MetropolisMonteCarlo. This somehow also seems to massively slow down the functionality of oneD_simulation_analysis_plot().