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AIRSS Workflow

1. Generate Files

Example: gencell 10 1 Rb 1 F 1

(Parameters like 10, 1, 1 can be changed later; first create the .cell file)

Parameters in order: - gencell: File generation command - 10: Preset volume - 1: Supercell multiplier - Rb, F: Element names composing the compound - 1, 1: Ratio of each element

This will generate two files: *.cell and *.param

2. Edit Files

(1) *.cell

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%BLOCK LATTICE_CART
2.714416 0 0
0 2.714416 0
0 0 2.714416

Diagonal values are the cube root of volume

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#VARVOL=20

Volume, search at https://uspex-team.org/online_utilities/volume_estimation/

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%BLOCK POSITIONS_FRAC
Rb 0.0 0.0 0.0 # Rb1 % NUM=0-7
F 0.0 0.0 0.0 # F1 % NUM=0-7
%ENDBLOCK POSITIONS_FRAC

NUM is the number of each atom in the formula

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##SPECIES=Rb,F
##NATOM=3-9
##FOCUS=2

Double ## represents comments; keeping only one # activates them. - First item: element types - Second item: total number of atoms - Third item: if activated, predicts structures containing pure elements Using the second item can delete the entire preceding %BLOCK LATTICE_CART parameter

(2) *.param

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cut_off_energy : 0 eV

Cutoff energy: Drag the generated .cell file into VESTA, then export as .cif file and drag into Material Studio. Note: must be an English path. Select the three wavy lines icon Calculation-Electronic, options:

For Energy cutoff, SCF tolerance, k-point set, select Fine.

For Pseudopotentials, select On the Fly or OTFG ultrasoft.

Then check the value next to Energy cutoff; this value is the *.param cut_off_energy value.

When resources allow, actual calculations generally tend to be slightly higher than that number. Higher values correspond to higher accuracy, but computation speed will be slower.

You can also use MS for Encut testing: build→symmetry→find symmetry→click and then click find symmetry in new window→impose symmetry

3. airss.pbs

You can copy from others' working directories and modify (recommended), or create your own, but it varies slightly between servers. Example:

Supercomputing Center:

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#!/bin/bash
#PBS -N castep
#PBS -l nodes=1:ppn=28
#PBS -j n
#PBS -q CT2
#PBS -e ${PBS_JOBNAME}.err
#PBS -o ${PBS_JOBNAME}.out
cd $PBS_O_WORKDIR
airss.pl -pressure 200 -mpinp 28 -max 500 -keep -seed RbF >> log

4. Results Analysis

Fixed cell:

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ca -u accuracy -r -t

Finally, find the corresponding structure and export using Xftp or Winscp.

Note: Accuracy generally can be between 0.1 - 0.5. As for selecting how many structures for high-precision optimization, there's no unified number; depends on the enthalpy difference between them. Within 0.5eV should be considered.

  • First column: Structure name label assigned by AIRSS software
  • Second column: Pressure value (GPa)
  • Third column: Volume per formula unit (fu)
  • Fourth column: First row is enthalpy per formula unit (fu); subsequent rows are enthalpy values relative to the first row under different structures
  • Fifth column: Total number of formula units (fu) in unit cell (number of fug in unit cell multiplied by number of fu in one fug)
  • Sixth column: Chemical formula of formula unit (fu)
  • Seventh column: Space group name
  • Eighth column: Number of times this structure appears in all search results

If you think there are too many listed results, you can use the -u option, but note that -u must be used before -r.

Variable cell:

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ca -R -m  # Add -R if in subfolder, don't add in root directory

Command to list structures on convex hull plot:

Fixed cell: 

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ca -r -de 0

Variable cell: 

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ca -m -de 0

(The 0 at the end is accuracy; if you input 0.1, it lists points within 0.1 error)

Plot convex hull: 

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xmgrace hull.agr  # Requires xmgrace installation

Additional Commands

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ca -m -de 0.05 --delete  # List points 0.05 away from convex hull and delete other .res files

After structure optimization:

  1. Copy res files found by airss structure search to another new folder called test
  2. Then input ca -m -de 0.04 -r --delete to delete res files with single atom energy difference greater than 0.04eV. The value 0.04 should vary based on actual situation
  3. Copy the cell and param files from structure search to this new folder test, increase K-points in cell, add pressure, increase cutoff energy in param file
  4. Use command run.pl -mpinp 8 -keep to continuously calculate all structures in folder test
  5. Compare enthalpy values of these structures; the lowest enthalpy is the most stable structure

Other commonly used commands:

First create tmp folder

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ca -m -de 0.05 |awk '{print $1".res"}' |xargs cp -t tmp/

Only list the most stable stoichiometry and copy to tmp folder

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ca -f *** -r -t|awk '{print $1".res"}' |xargs cp -t tmp/

Find the top ten stable structures of a certain stoichiometry and copy to tmp folder

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ca --m | sort --n --k 6 --k 5

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pestat  # View currently executing tasks