compute_angular_rdf

This keyword is used to compute the angular-dependent radial distribution function (ARDF) for all atoms or pairs of species. It works for classical MD. The results will be written to the angular_rdf.out file.

Mathematical details

The ARDF is defined as

\[g (r, \theta) = \frac{n (r, \theta)}{\rho 2 r^2 \Delta r \Delta \theta}\]

, where \(n (r, \theta)\) is the number of pairs of atoms at a distance \((r-\Delta r/2, r+\Delta r/2]\) and an angle \((\theta-\Delta \theta/2, \theta+\Delta \theta/2]\) from each other, \(\rho\) is the number density, \(r\) is the distance between the atoms, \(\theta\) is the angle between the atoms, \(\Delta r\) is the bin width in distance, and \(\Delta \theta\) is the bin width in angle.

Syntax

This keyword is used as follows:

compute_angular_rdf <cutoff> <r_num_bins> <angular_num_bins> <interval> [atom <i1> <i2> atom <i3> <i4> ...]

This means that the ARDF calculations will be performed every interval steps, with r_num_bins data points evenly distributed from 0 to cutoff (in units of Ångstrom) in terms of the distance between atom pairs, and angular_num_bins data points evenly distributed from -Pi/2 to Pi/2 in terms of the angle.

Without the optional parameters, only the total ARDF will be calculated.

To additionally calculate the partial ARDF for a pair of species, one can specify the types of the two species after the word “atom”. The types 0, 1, 2, … correspond to the species in the potential file in order. Currently, one can specify at most 6 pairs.

Example

compute_angular_rdf 8.0 400 100 1000 # total ARDF every 1000 MD steps with 400 bins in distance and 100 bins in angle up to 8 Angstrom compute_angular_rdf 8.0 400 100 1000 atom 0 0 atom 1 1 atom 0 1 # additionally calculate 3 partial ARDFs