[gmx-developers] Trajectory Analysis: Problem with accessing velocities of positions in a selection

[Tobias Pfeiffer]

Hi everyone,

this is my first post to this email list and I am not sure, if it is the right place to do so. If this is not the case I want to apologize for wasting everyones time, but here we go:

I spent the last days with writing some custom trajectory analysis tools based on the provided file template.cpp. Everything worked well until the time I wanted to access the velocities of my positions in a selection. 

Every time I use the function gmx::SelectionPosition::v() to access the velocity of a position the return value is zero. Of course I added the option evaluateVelocities() to the selection and checked if there are velocities available in this selection using Selection::hasVelocities() before calling v(). I am also positive that the velocities in the provided trajectory are non-zero. Still the function returns zero in every frame of the trajectory and for every position in the selection.

Most probably this is due to a stupid error in my program code, but as there is no working example I know of I am not able to solve this problem on my own. Instead of discussing my program in unnecessary depth, I just ask the following question that simplifies the problem:

What are all the necessary changes in the template for the custom trajectory analysis tool to access the velocity of specific positions in the reference and the selection using the function gmx::SelectionPosition::v()?
 
Every help is appreciated. Thanks in advance

Tobias
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