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Introduction

Before any MD simulations can be run we require a suitable MM force field (potential function). For Ionic Liquids there are several MM force fields available. In our work we currently use the Imidazolium based IL FF of Can-Lopes et al.

Specific example using [EMIM]+[AlCl4]-

  • For this example, we refer to these two articles for force field parameters[1][2].
  • In the reported articles, the AMBER force field functional form for the potential energy V is used:
  • ε is the well depth and a measure of how strongly the two particles attract each other.  
  • σ is the distance at which the intermolecular potential between the two particles is zero.
  • σ also gives a measure of how close two nonbonding particles can get and is thus referred to as the Van der Waals radius. It is equal to one-half of the internuclear distance between nonbonding particles.
  • And lastly, r is the distance of separation between both particles (measured from the center of one particle to the center of the other particle).


  • Start with 3 input files: CONFIG, CONTROL and FIELD
  • CONFIG gives your initial configuration (obtained from packmol and converted), CONTROL gives your parameters, and FIELD provides the force field
  • Sample input file FIELD:
EMIM-ALCl4 - JPCB 2002 v.106 p.3546
units kJ
molecules 2
EMIM
nummols 128
atoms 19
CR     12.011000000  -0.11000 1
NA     14.006740000   0.15000 1
CW     12.011000000  -0.13000 1
CW     12.011000000  -0.13000 1
NA     14.006740000   0.15000 1
HCR     1.007940000   0.21000 1
HCW     1.007940000   0.21000 1
HCW     1.007940000   0.21000 1
C1     12.011000000  -0.17000 1
H1      1.007940000   0.13000 1
H1      1.007940000   0.13000 1
H1      1.007940000   0.13000 1
C1     12.011000000  -0.17000 1
H1      1.007940000   0.13000 1
H1      1.007940000   0.13000 1
C2     12.011000000  -0.05000 1
HC      1.007940000   0.06000 1
HC      1.007940000   0.06000 1
HC      1.007940000   0.06000 1
bonds 8
harm    1     2     3992.00000   1.3150
harm    1     5     3992.00000   1.3150
harm    2     3     3574.00000   1.3780
harm    5     4     3574.00000   1.3780
harm    2     9     2820.00000   1.4660
harm    5    13     2820.00000   1.4660
harm    3     4     4352.00000   1.3410
harm    13   16     2242.00000   1.5290
constrains 11
   1     6     1.080000
   4     8     1.080000
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  • Explanation of above snapshot:
  • scrolling down...
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angles 33
harm   5   1   2   585.20   109.8000
harm   6   1   2   292.60   125.1000
harm   6   1   5   292.60   125.1000
harm   1   2   3   585.20   108.0000
harm   1   5   4   585.20   108.0000
harm   2   3   7   292.60   122.0000
harm   5   4   8   292.60   122.0000
harm   2   3   4   585.20   107.1000
harm   5   4   3   585.20   107.1000
harm   8   4   3   292.60   130.9000
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  • Explanation of above snapshot:
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dihedrals 46
cos3     6     1     2     9    0.000000   19.460000   0.000000   0.500000   0.500000
cos3     6     1     5    13    0.000000   19.460000   0.000000   0.500000   0.500000
cos3     6     1     2     3    0.000000   19.460000   0.000000   0.500000   0.500000
cos3     6     1     5     4    0.000000   19.460000   0.000000   0.500000   0.500000
cos3     1     2     3     7    0.000000   12.550000   0.000000   0.500000   0.500000
cos3     1     5     4     8    0.000000   12.550000   0.000000   0.500000   0.500000
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  • Explanation of above snapshot:

Harmonic Dihedral Potential Vn/2 (1 + cos[nφ − γ]) from forcefield functional

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cos3     5     3     4     8    0.000000    9.200000   0.000000   0.500000   0.500000
cos3     2     4     3     7    0.000000    9.200000   0.000000   0.500000   0.500000
finish
alumcl
nummols 128
atoms 5
AL     26.9815400000   0.90920 1
CL     35.4530000000  -0.47730 4
bonds 4
harm    1     2      485.34000   2.1700
harm    1     3      485.34000   2.1700
harm    1     4      485.34000   2.1700
harm    1     5      485.34000   2.1700
angles 6
harm   3   1   2   209.20   109.5000
harm   4   1   2   209.20   109.5000
harm   5   1   2   209.20   109.5000
harm   4   1   3   209.20   109.5000
harm   5   1   3   209.20   109.5000
harm   5   1   4   209.20   109.5000
finish
vdW 66
CR    CR    lj    0.29288000 3.55000000
CR    NA    lj    0.45642051 3.39669000
CR    CW    lj    0.29288000 3.55000000
CR    HCR   lj    0.19173497 2.93104100
CR    HCW   lj    0.19173497 2.93104100
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  • Explanation of above snapshot:

Notes

  1. ↑ J. D. Andrade, E. S. Boes, H. Stassen, J. Phys. Chem. B, 2002, 106, 3546-3548
  2. ↑ T. Fox, P. A. Kollman, J. Phys. Chem. B, 1998, 102, 8070-8079

References

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