# Calculating the x, y and z distances between two molecules - AMBER (Molecular Dynamics)

I am interested in calculating the x,y and z distance between two molecules over a given simulation time. I know how to calculate the total (r) distance between them, but I want the distance between them along all three dimensions.

For the total distance, cpptraj can be used as:

parm prmtop-file
trajin netcdf-file
distance DIST :1-4@heavy-atoms-from-molecule-1 :5-8@heavy-atoms-from-molecule-2 out dist.out
run
quit


But like I said before, I do not want the r-distance. Is there anyway to calculate the x,y and z-distances in cpptraj? If not, what other alternatives do I have?

• I'm not familiar with AMBER, but in LAMMPS there is a file containing all the atom positions and a utility to convert it to a .xyz file which is just a list of all the coordinates. You might try searching for something like an .xyz file? – ericksonla Jun 5 '18 at 14:43
• I can find the coordinates via a file in AMBER called a .pdb file, but I will have to manually calculate the distances from this, which is over 20000 frames to consider. I guess I might as well just do it. – LLCJ27 Jun 6 '18 at 4:30

I ended up realizing that if you run:

parm prmtop-file
trajin netcdf-file
vector v0 :1-4@heavy-atoms-from-molecule-1 :5-8@heavy-atoms-from-molecule-2 out v0.out magnitude
run
quit


You will get the total distance as well as x, y and z distances between your two molecules. The output file will contain 8 columns. The AMBER manual states that they are identified as follows:

Frame | x-Vector-Coordinate | y-Vector-Coordinate | z-Vector-Coordinate | x-Origin-Coordinate | y-Origin-Coordinate | z-Origin-Coordinate | Magnitude

The vector coordinates are your x, y and z distances. Magnitude is the total distance.

Decided to share this as someone might run into the same problem one day.