# Input file¶

The input file, also known as a .bod file, contains the description of the object and some additional input parameters.

## Defining the object(s)¶

A single object of interest must be described by a collection of spheres and cuboids, which may or may not be overlapping. The code has also be extended for the case of running not just a single snapshot composed of a collection of spheres, but also a trajectory or series of snapshots each composed of a collection of spheres. In all cases the shape of the object is defined in the .bod file.

### Spheres¶

Spheres are defined by lines of the form

SPHERE x y z r


where x, y, and z are the coordinates of the center of the sphere and r is the radius.

For example, a .bod file that contains the following describes an object composed of two spheres: one of radius 2 at $$x=0$$, $$y=0$$, and $$z=1$$ and one of radius 3 at $$x=0$$, $$y=0$$, and $$z=-1$$.

SPHERE 0 0 1 2
SPHERE 0 0 -1 3


### Cuboids¶

Cuboids can be defined in several ways. The most basic is a line of the form

CUBOID x1 y1 z1 x2 y2 z2


where x1, y1, and z1 are the coordinates of one corner of the cuboid and x2, y2, and z2 are the coordinates of the opposite corner. The edges of the cuboid are aligned with the $$x, y, z$$ axes.

A cuboid with all edges the same length is a cube. Cubes can be defined with lines of the form

CUBE x y z L


where x, y, and z are the coordinates of one corner of the cube and L is the edge length. This is equivalent to

CUBOID x y z x+L y+L z+L


For example, a .bod file that contains the following describes an object composed of two cuboids: one with a corner at $$x=0, y=0, z=0$$ and opposite corner at $$x=1, y=2, z=3$$ and one with a corner at $$x=1, y=0, z=0$$ and opposite corner at $$x=5, y=4, z=4$$.

CUBOID 0 0 0 1 2 3
CUBE 1 0 0 4


Finally, sets of cuboids can be defined in a binary file in the .fits.gz format [1] using the voxels command. Voxels are specified with lines of the form

VOXELS <relative path to .fits.gz file>


Paths to the .fits.gz file are relative to the location of the .bod file. So, for example, if you had a voxels file voxels.fits.gz in the same directory as the .bod file, you could simply specify it as

VOXELS voxels.fits.gz


### Multiple snapshots or trajectories of spheres¶

In order to be compatible with a variety of existing software packages, the trajectories of spheres are defined using the xyz file format and referenced in the .bod file. The format of the xyz file is

<number of atoms>
comment line
<atom type> <x> <y> <z>
...


where atom type can be either a number or string, such as an element symbol. This structure can be repeated multiple times for multiple snapshots. For example,

2
snapshot 1
A -1 0 0
B 0.25 0 0
1
snapshot 2
A 0 0 0


would define two spheres of different types for the first snapshot and one sphere for the second snapshot where that sphere is the same type as the first sphere in the first snapshot. As the xyz file format does not contain radii information, a second conversion file that defines the radius of each atom type is needed. The conversion file format is

<atom type> <radius>


Each atom type in the xyz file must be defined. A corresponding conversion file for the xyz file example could be

A 1
B 0.25


In this case, together the two examples define a system of two touching spheres one of radius 1 and one of radius 1/4 for the first snapshot and a single sphere of radius 1 for the second snapshot.

The xyz file and the conversion file are specified in the .bod file as

TRAJECTORY <relative path to xyz file> <relative path to conversion file>


Note that if a trajectory is given, no other geomerty may be included in the .bod file.

## Optional inputs¶

 Command: rlaunch double Explanation: Sets the radius, which is radius of the sphere from which random walks are launched. The radius must be large enough to enclose the entire object. Default value: The smallest radius that encloses the smallest axis-aligned bounding-box of the object. Example: rlaunch 20 means that the launch radius is 20.

### Skin thickness¶

 Command: st double Explanation: Sets the skin thickness. A random walker is assumed to have hit the surface of the object if the distance between the surface and the walker is less than the skin thickness. Default value: 1e-6 times the launch radius Example: st 0.01 means that the skin thickness is 0.01.

### Units for length¶

 Command: hunits double string Explanation: Specifies the units for the length for all objects. Options: The string can take the following values: m (meters) cm (centimeters) nm (nanometers) A (Angstroms) L (generic or unspecified length units) Default value: 1 L Example: hunits 10 cm means that a length of 1 for an object is equivalent to 10 cm.

### Temperature¶

 Command: temp double string Explanation: Specifies the temperature, which is used for computing the diffusion coefficient. Options: The string can take the following values: C (Celsius) K (Kelvin) Default value: None Example: temp 20 C means that the temperature is 20$$^\circ$$C.

### Mass¶

 Command: mass double string Explanation: Specify the mass of the object, which is used for computing the intrinsic viscosity in conventional units and the sedimentation coefficient. Options: The string can take the following values: Da (Daltons) kDa (kiloDaltons) g (grams) kg (kilograms) Default value: None Example: mass 2 g means that the mass of the object is 2 grams.

### Solvent viscosity¶

 Command: viscosity double string Explanation: Specify the solvent viscosity, which is used for computing the diffusion coefficient, the friction coefficient, and the sedimentation coefficient. Options: The string can take the following values: p (poise) cp (centipoise) Default value: None Example: viscosity 2 cp means that the solvent has a viscosity of 2 centipoise.

### Buoyancy factor¶

 Command: bf double Explanation: Specify the buoyancy factor, which is used for computing the sedimentation coefficient. Default value: None Example: bf 2 means that the buoyancy factor is 2.