Messages

Hello RASPA community,

I wonder if enthalpy of adsorption of individual components in a binary mixture can be reliably computed using the fluctuation formula?

I am simulating a system for competitive adsorption of CH4 and N2 in NaY zeolites using the GCMC technique. Although RASPA can calculate "total enthalpy of adsorption" correctly, it returns "-nan" values for enthalpy of adsorption of each component in the mixture (i.e. CH4 and N2).

I wonder if anyone has a clue why this is the case? Is this a RASPA problem or the fluctuation formula cannot be used for this purpose?

Many thanks in advance.
     

Dear RASPA community,

I wonder if anyone has attempted to compute PSD for a porous framework with mobile cations (e.g. cation-exchanged zeolites)?
I have tried to do this for NaY zeolite with mobile Na cations. I have successfully set up a MC simulation that runs perfectly and computes PSD at every few steps (please see an example of my input file below ).

However, I wonder if RASPA takes into account mobility of cations in its calculation of the PSD in such cases? (i.e. PSD must be slightly different every time that cations move around and be averaged at the end).

My input file:
==============
SimulationType      MC
NumberOfCycles      10000
NumberOfInitializationCycles 0
PrintEvery              100
PrintPropertiesEvery    100
RestartFile             yes

Forcefield           local
CutOffVDW              24.0
ChargeMethod           Ewald 
CutOffChargeCharge     24.0
EwaldPrecision         1e-6


Framework 0
FrameworkName  NaY
UseChargesFromCIFFile   yes
UnitCells 2 2 2
ExternalTemperature 298

Movies yes
WriteMoviesEvery 100

ComputePSD                yes
PSDProbeDistance          Sigma
WritePSDHistogramEvery    100
PSDHistogramSize          100
PSDRange                  10


Component 0 MoleculeName                    Na
                    MoleculeDefinition              Na
                    TranslationProbability          1.0
                    ExtraFrameworkMolecule          yes
                    CreateNumberOfMolecules         0

====================================

Thank you in advance.

You are running a simulation in NPT ensemble which means the volume of your simulation box is no longer constant. The unit cell can change in shape and size. Please check dimensions of your unit cell (a, b, c, alpha, beta, gamma) at every step, this way you can confirm how volume is changing.

Hope it helps.
Cheers,

Thanks David for your reply.
Much appreciated.

You first need to compile and install RASPA according to the procedure explained in the User Guide (section 1.3 Compiling and installing RASPA).
As part of this procedure, you will need to set the RASPA_DIR environment variable: export RASPA DIR=${HOME}/RASPA/simulations/
This is how you will create this directory which also contains the executable file.

Hope this helps.

Hello everyone,

I have two questions for the RASPA community regarding the use of grid potentials in MC/GCMC simulations:

(1) I am simulating adsorption of CH4 and N2 in NaY cation-exchanged zeolites which contain mobile Na+ cations. I was wondering if grid potentials in RASPA can be also used for mobile cations (such as Na+) in the same way that one would use them for neutral adsorbate molecules? In fact, I have created a grid potential for Na+ in my system, however when I run a GCMC simulation using the corresponding grid file, I get the following error from RASPA in the output file where it tests the accuracy of grids:

PseudoAtom 7 Framework-[Na]
=========================================================================================
        Boltzmann average energy VDW (table)                 :     0.000000000000
        Boltzmann average energy VDW (full)                  :               -nan
        Boltzmann relative error VDW                         :     0.000000000000
        Boltzmann average energy Coulomb (table)             : 3091865.680709035601
        Boltzmann average energy Coulomb (full)              :               -nan
        Boltzmann relative error Coulomb                     :               -nan
=========================================================================================
        Boltzmann average Force(x) VDW (table)               :     0.000000000000
        Boltzmann average Force(x) VDW (full)                :               -nan
        Boltzmann relative error VDW                         :     0.000000000000
        Boltzmann average Force(x) Coulomb (table)           : 90829.808190701209
        Boltzmann average Force(x) Coulomb (full)            :               -nan
        Boltzmann relative error Coulomb                     :               -nan
=========================================================================================
        Boltzmann average Force(y) VDW (table)               :     0.000000000000
        Boltzmann average Force(y) VDW (full)                :               -nan
        Boltzmann relative error VDW                         :     0.000000000000
        Boltzmann average Force(y) Coulomb (table)           : -410892.822263886570
        Boltzmann average Force(y) Coulomb (full)            :               -nan
        Boltzmann relative error Coulomb                     :               -nan
=========================================================================================
        Boltzmann average Force(z) VDW (table)               :     0.000000000000
        Boltzmann average Force(z) VDW (full)                :               -nan
        Boltzmann relative error VDW                         :     0.000000000000
        Boltzmann average Force(z) Coulomb (table)           : 25639.732840344324
        Boltzmann average Force(z) Coulomb (full)            :               -nan
        Boltzmann relative error Coulomb                     :               -nan

This is bizarre to me because in my system, Na+ interacts with the framework using both BUCKINGHAM2 and coulomb interactions.
So, why Boltzmann average energy for VDW is either zero or nan? Why Boltzmann average energy (full) for Coulomb is nan?

(2) I also wonder if one could model some of the interatomic interactions using grids, while the rest of them are modelled directly (without any grids) in the same simulation run?

Any assistance will be much appreciated.
Thank you.

Dear David,

Many thanks for your response.
Are you suggesting that RASPA implements the EQeq method (extended version of Qeq) rather than the original Qeq technique developed by Rappé and Goddard?

Kind regards,
AHF

Hello RASPA community,

I am trying to reproduce partial charges of BEA zeolite using the Qeq method. My reference is Table 2 of the following paper by Wolffisa et al:  https://www.sciencedirect.com/science/article/abs/pii/S1387181118305614

In this paper, they have used Qeq method as implemented in GULP, although I am using RASPA for this purpose. I am using the exact same structure, however I still get results that are very different from what is reported in this paper. I have also tested different oxidation states but still no success.

I was wondering if there is any other parameters to tune in RASPA in order to reproduce the above partial charges? Could the observed discrepancy be simply due to different implementation of Qeq method in GULP and RASPA?

Any help will be greatly appreciated.
Best regards,
AHF