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gezelter |
1342 |
Madelung Energy Sample Calculation |
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The sample in this directory provides a way of checking the value of |
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the Madelung Energy for a perfect crystal of NaCl. The relevant |
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quantities are: |
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M (Madelung constant) = 1.74756 |
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a (lattice constant) = 5.65 Angstroms |
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q^2 / (4 pi e0 a) = 58.77233 kcal / mol |
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M q^2 / (4 pi e0 a) = 102.708173 kcal / mol |
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The file NaCl.md contains 8000 ions, so the total electrostatic energy |
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of the perfect crystal in this file should be: |
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V_electrostatic = -821665.38 kcal / mol |
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Using different electrostatic calculation methods, we can get quite |
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close to this value. |
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For example, with : |
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electrostaticSummationMethod = "shifted_force"; |
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electrostaticScreeningMethod = "damped"; |
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cutoffRadius = 28; |
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dampingAlpha = 0.14159292; |
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switchingRadius = 28; |
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The resultant electrostatic potential is: -821667.88 kcal / mol |
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To obtain values for the electrostatic potential in OOPSE, we add the |
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ELECTROSTATIC_POTENTIAL keyword to the end of the statFileFormat: |
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statFileFormat = "TIME|TOTAL_ENERGY|POTENTIAL_ENERGY|KINETIC_ENERGY|TEMPERATURE|PRESSURE|VOLUME|CONSERVED_QUANTITY|ELECTROSTATIC_POTENTIAL"; |
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Note: Converging the Madelung energy often requires quite the use of |
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very large cutoff distances (on the order of 28 Angstroms). This can |
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result in extremely large neighbor lists, well beyond the size |
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normally encountered by OOPSE. If you are interested in Madelung |
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energy calculations, you may need to recompile OOPSE after making |
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changes to the following parameters in |
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src/UseTheForce/DarkSide/neighborLists.F90 : |
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integer, save :: listMultiplier = 200 |
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integer, parameter :: maxAllocations = 25 |
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This recompilation should not be required in future versions of OOPSE. |
