# Options

There are some options to control what exactly is going to be computed to obtain the MDDF. These options can be defined by the user and passed to the mddf function, using, for example:

options = Options(lastframe=1000)
results = mddf(trajectory,options)

### Most common options that the user might want to control are:

firstframe: Integer, first frame of the trajectory to be considered.

lastframe: Integer, last frame of the trajectory to be considered.

stride: Integer, consider every stride frames, that is, if stride=5 only one in five frames will be considered.

binstep: Real, length of the bin step of the histograms, default = 0.02 Angstroms.

dbulk: Real, distance from which the solution is to be considered as a bulk solution, that is, where the presence of the solute does not affect the structure of the solution anymore. This parameter is important in particular for systems with a single solute molecule (a protein, for example), where the density of the solvent in the box is not the bulk density of the solvent, which must be computed independently. Default: 10 Angstroms.

cutoff: Real, the maximum distance to be considered in the construction of histograms. Default: 10 Angstroms.

usecutoff: true/false: If true, the cutoff distance might be different from dbulk and the density of the solvent in bulk will be estimated from the density within dbulk and cutoff. If false, the density of the solvent is estimated from the density outside dbulk by exclusion. Default: false.

### Options that most users will probably never change:

irefatom: Integer, index of the reference atom in the solvent molecule used to compute the shell volumes and domain volumes in the Monte-Carlo volume estimates. The final rdf data is reported for this atom as well. By default, we choose the atom which is closer to the center of coordinates of the molecule, but any choice should be fine.

n_random_samples: Integer, how many samples of random molecules are generated for each solvent molecule to compute the shell volumes and random MDDF counts. Default: 10. Increase this only if you have short trajectory and want to obtain reproducible results for that short trajectory. For long trajectories (most desirable and common), this value can even be decreased to speed up the calculations.

seed: Seed for random number generator. If -1, the seed will be generated from the entropy of the system. If your results are dependent on the seed, is is probable that you do not have enough sampling. Mostly used for testing purposes. Two runs are only identical if ran with the same seed and in serial mode.

StableRNG (::Bool), defaults to false. Use a stable random number generator from the StableRNGs package, to produce identical runs on different architectures and Julia versions. Only used for testing.

nthreads: How many threads to use. By default, all available threads will be used (for multi-threading, one for managing, n-1 for computation).

lcell: Integer, the cell length of the linked-cell method (actually the cell length is cutoff/lcell). Default: 2.

sleep: Real, the time between checks between multiple spawns of calculations in parallel. Default 0.1 s.

GC: Bool, force garbage collection in parallel runs, to avoid memory overflow. Default: true. That this might be required is probably a result of something that can vastly improved in memory management. This may slow down parallel runs significantly if the GC runs too often.

GC_threshold: Float64, minimum fraction of the total memory of the system required to force a GC run. That is, if GC_threshold=0.1, which is the default, every time the free memory becomes less or equal to 10% of the total memory available, a GC run occurs.