CP2K Open Source Molecular Dynamics

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gpw [2014/02/10 11:54] – [ot / diag] oschuettgpw [2018/02/16 12:40] – [Cutoff] 195.19.236.218
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 $n$ and $\tilde n$ are not equal, and this introduces an error in the calculation. $\tilde n$ converges toward $n$ when the cutoff (that controls the grid spacing) goes to infinity (and gridspacing to 0). Which cutoff is sufficient to represent a density depends on how sharp is the gaussian basis set (or that of the potential, but it is always broader). $n$ and $\tilde n$ are not equal, and this introduces an error in the calculation. $\tilde n$ converges toward $n$ when the cutoff (that controls the grid spacing) goes to infinity (and gridspacing to 0). Which cutoff is sufficient to represent a density depends on how sharp is the gaussian basis set (or that of the potential, but it is always broader).
  
-For historical reasons the density of the grid is given as the energy (in Ry) of the highest reciprocal vector that can be represented on the on the grid. This can be roughly given as $0.5(\pi/dr)^2$ where $dr$ is the gridspacing in Bohr. The characteristic length of a gaussian with exponent A is given by $1/\sqrt{a}$ (up to a factor 2 depending on the convention used). This means that the cutoff to represent in the same way a gaussian depends linearly on the exponent. Thus one can get a first guess for an acceptable guess can be take from the knowledge that for water with $\alpha_H=47.7$ a good cutoff for doing MD is 280 Ry.+For historical reasons the density of the grid is given as the energy (in Ry) of the highest reciprocal vector that can be represented on the grid. This can be roughly given as $0.5(\pi/dr)^2$ where $dr$ is the gridspacing in Bohr. The characteristic length of a gaussian with exponent A is given by $1/\sqrt{a}$ (up to a factor 2 depending on the convention used). This means that the cutoff to represent in the same way a gaussian depends linearly on the exponent. Thus one can get a first guess for an acceptable guess can be take from the knowledge that for water with $\alpha_H=47.7$ a good cutoff for doing MD is 280 Ry.
  
 It turns out that if one wants to put the whole density on the grid, the core electrons of even the simplest atoms cannot be represented, thus one has to remove to core electrons and use pseudopotentials for the atoms. In Cp2k we use the [[GTH_pseudopotentials|Godeker-Theta-Hutter pseudopotentials]], these are harder than other pseudopotentials, but also more accurate. It turns out that if one wants to put the whole density on the grid, the core electrons of even the simplest atoms cannot be represented, thus one has to remove to core electrons and use pseudopotentials for the atoms. In Cp2k we use the [[GTH_pseudopotentials|Godeker-Theta-Hutter pseudopotentials]], these are harder than other pseudopotentials, but also more accurate.
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 <code> <code>
-&XC_FUNCTIONAL BLIP+&XC_FUNCTIONAL BLYP
 &END XC_FUNCTIONAL &END XC_FUNCTIONAL
 </code> </code>
gpw.txt · Last modified: 2020/08/21 10:15 by 127.0.0.1