# 4. Settings for RASPA¶

The RASPA package can perform MC and MD calculations, for a variety of systems.

In this tutorial we use this program to perform Grand Canonical (GCMC) calculations in nanoporous structures. Here it is basic input:

SimulationType                MonteCarlo
NumberOfInitializationCycles  ***
NumberOfCycles                ***
PrintEvery                    ***

RestartFile                   no
RestartStyle                  Raspa

CutOff                        ***

Forcefield                    UFF-TraPPE

ChargeMethod                  ***

Framework 0
FrameworkName                 ***
UnitCells                     * * *

ExternalTemperature           ***
ExternalPressure              ***

Component 0 MoleculeName             methane
MoleculeDefinition       TraPPE
BlockPockets             yes/no
BlockPocketsFileName     *****
TranslationProbability   ***
RotationProbability      ***
ReinsertionProbability   ***
SwapProbability          ***
CreateNumberOfMolecules  ***


These settings are imported in AiiDA as a ParameterData object, with the “shape” of a python dictionary.

You should fill the input settings, replacing asterisks with meaningful values.

Consider that the forcefield parameters that you will be using to model the framework-molecule interactions are on your remote computer:

• $RASPA_DIR/share/raspa/forcefield/UFF-TraPPE/ contains the Lennard-Jones parameters for the framework atoms, from UFF, and for methane, from TraPPE • $RASPA_DIR/share/raspa/molecules/TraPPE/ contains the file methane.def with the critical T and P and acentric factor of methane (why?) and the geometry of this gas molecule. Note that we are using a forcefield where methane is modelled as a single point!

As an extra exercise you may decide to use another popular forcefield for the framework atoms: DREIDING. Note that, since it does not contain all the atoms of the periodic table, it is common practice to use UFF parameters for the missing ones.\ You can find this forcefield as $RASPA_DIR/share/raspa/forcefield/DREIDING-TraPPE/, and remember to modify Forcefield DREIDING-TraPPE in the input. You can even modify, create or import your own forcefield: just create the folder $RASPA_DIR/share/raspa/forcefield/{ffname}/, copy the file formatting of the other forcefield .def files and specify Forcefield {ffname} in the input.

## 4.1. A few hints for the settings:¶

• BlockPockets and BlockPocketsFileName will be filled by AiiDA: if Zeo++ finds some non accessible pore volume, it can generate a .block file with the positions and the radii of blocking spheres. These spheres are inserted in the framework to prevent Raspa from inserting molecules in the non accessible pore.

• You need to choose NumberOfInitializationCycles and NumberOfCycles. Consider that for every cycle every molecule of your system is selected to perform a MC movement that may be accepted or rejected. If your system is empty or there are a few particles, Raspa tries to perform GCMC insertions (“swaps”) to fill your system (see UMS, Algorithm 13). The number of particles continues to fluctuate according to the Grand Canonical ensemble, and consider that after NumberOfInitializationCycles cycles, Raspa start to compute the average number of particles for NumberOfCycles cycles. Therefore, the number of cycles should be large enough to compute statistics correctly but not too large to waist computational times after your uncertainty is already low. Standard deviation is computed using Block Averages (see UMS pag. 529).

Fig. 4.3 Here an example of the value fluctuation and average, with the averaging starting after 10k cycles.

• You have to chose the probability for the different swap movements. The total will be normalized: for example if you assign 2.0, 1.0, 1.0, 1.0, this will be normalized as 40%, 20%, 20%, 20% probability for each move.

• TranslationProbability: rigid displacement by a random distance. The maximum displacement is scaled during the simulation to achieve an acceptance ratio of 50%.

• RotationProbability: rotation around its starting bead

• ReinsertionProbability: reinsertion of the particle in a random position of the unit cell.

• SwapProbability: insertion or deletion move. Whether to insert or delete is decided randomly with a probability of 50% for each. The swap move imposes a chemical equilibrium between the system and an imaginary particle reservoir for the current component.

• Choose a reasonable cutoff (Angstrom) to exclude negligible Lennard-Jones interaction between far particles. Consider that the higher the cutoff the more you will have to expand your structures!

• You can choose ChargeMethod to None or Ewald, depending if you want or not to compute Coulombic interactions.

• FrameworkName will be filled by AiiDA

• UnitCells will be filled by the multiply_unit_cell(cif) function: it gives the cell expansion that you need to perform according to the cutoff you chose.

• With CreateNumberOfMolecules you can specify the number of particles that will be inserted (randomly) in your system at the start of your simulation.