Position dependent diffusivity of a molecule under an external harmonic potential. The method is based on the derivation given in the reference below.

Hummer, Gerhard. "Position-dependent diffusion coefficients and free energies from Bayesian analysis of equilibrium and replica molecular dynamics simulations." New Journal of Physics 7.1 (2005): 34.

Compilation:

gfortran -o diffusion diffusion.f90

The code takes in four command line arguments:

1. Autocorrelation cutoff time (in ps)
2. Time step between frames (in ps)
3. Trajectory file name (two column ASCII)
4. Output name for ACF results

The trajectory should be formatted as a two column file with the time/frame number in the first and the position in the second. The positions should be in units of Angstroms. Running the program with

./diffusion 2.0 0.001 wind01.traj acf01.txt

should give an output file acf01.txt. The output file contains the some information in the header followed by the autocorrelation function.

# Frame time (ps)          :  0.00100
# Correlation time (ps)    :  2.00000
# Characteristic time (ps) :  0.24332
# Diffusion (x10^-9 m^2/s) :  1.25022
   0.00100   1.18259
   0.00200   1.18151
   0.00300   1.18012
   0.00400   1.17821
   0.00500   1.17590
   0.00600   1.17310
   0.00700   1.16977
   0.00800   1.16591
   ...