+++
title = "Mesocrystal summation"
weight = 20
+++

## Option: mesocrystal computation

Scattering on mesocrystal can be computed in two ways.

1. The first and the default way is to find all the lattice position inside
the mesocrystal shape and coherently sum the scattering from all the unit cells.
This gives the exact result, equal to the composition of all these particles.
The computation time depends on the number of particles in mesocrystal.

2. The second way is to wrap form-factor of the mesocrystal outer shape
with summation over the reciprocal lattice nodes within a certain reasonable radius.
The result is approximate; the artifacts are stronger for small outer shapes.
The computation time does _not_ depend on the number of particles in mesocrystal.

```python
simulation.options().setMesoReciprocalSum(True, 2.1)
```
The default state is `False` which corresponds to the first approach.

The second argument is the factor determining the summation radius of
reciprocal lattice. The computation time grows as cube of this factor.

