gpw

# Differences

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 gpw [2018/02/16 12:40]195.19.236.218 [Cutoff] gpw [2018/02/16 13:00] (current)195.19.236.218 [Smoothing] Both sides previous revision Previous revision 2018/02/16 13:00 [Smoothing] 2018/02/16 12:40 [Cutoff] 2016/10/18 12:53 2014/02/10 11:54 oschuett [ot / diag] 2014/02/09 14:31 oschuett converted from doc/tutorialCp2k.html, Credit: Fawzi Mohamed 2018/02/16 13:00 [Smoothing] 2018/02/16 12:40 [Cutoff] 2016/10/18 12:53 2014/02/10 11:54 oschuett [ot / diag] 2014/02/09 14:31 oschuett converted from doc/tutorialCp2k.html, Credit: Fawzi Mohamed Line 17: Line 17: ===== Smoothing ===== ===== Smoothing ===== $\tilde n$ is optimized for the electrostatic part, but is used also to calculate the exchange and correlation potential. Because of this, and because the [[GTH pseudopotential]] goes almost to 0 close to the core of the atom, the xc potential, especially for gradient corrected functionals,​ converges badly. Instead of using very high cutoffs one can perform a smoothing of the density, and calculate the derivatives on the grid with other methods than the G-space based derivatives. $\tilde n$ is optimized for the electrostatic part, but is used also to calculate the exchange and correlation potential. Because of this, and because the [[GTH pseudopotential]] goes almost to 0 close to the core of the atom, the xc potential, especially for gradient corrected functionals,​ converges badly. Instead of using very high cutoffs one can perform a smoothing of the density, and calculate the derivatives on the grid with other methods than the G-space based derivatives. - For MD of water using a cutoff of 280 Ry ''​XC_SMOOTH_RHO NN10''​ and ''​XC_DERIV SPLINE2_SMOOTH''​ (in the ''​FORCE_EVAL%DFT%SC%XC_GRID''​ section) give good results, please note that these options renormalize the total energy, and the amount of renormalization is dependent on the cutoff. Thus energies with different cutoffs cannot be easily compared, only interaction energies or forces can be calculated. + For MD of water using a cutoff of 280 Ry ''​XC_SMOOTH_RHO NN10''​ and ''​XC_DERIV SPLINE2_SMOOTH''​ (in the ''​FORCE_EVAL%DFT%XC%XC_GRID''​ section) give good results, please note that these options renormalize the total energy, and the amount of renormalization is dependent on the cutoff. Thus energies with different cutoffs cannot be easily compared, only interaction energies or forces can be calculated. Methods that do not redefine the total energy are ''​XC_SMOOTH_RHO NONE''​ and ''​XC_DERIV''​ equal to either ''​PW,​ SPLINE3''​ or ''​SPLINE2''​. These are listed from the one that assumes more regularity (''​PW''​ the the one that assumes less regularity ''​SPLINE2''​. Normally ''​SPLINE2''​ is a good choice, but for high cutoffs (600 Ry for water) ''​SPLINE3''​ is better. The default (''​PW''​) is not bad, but generally inferior to the others. Methods that do not redefine the total energy are ''​XC_SMOOTH_RHO NONE''​ and ''​XC_DERIV''​ equal to either ''​PW,​ SPLINE3''​ or ''​SPLINE2''​. These are listed from the one that assumes more regularity (''​PW''​ the the one that assumes less regularity ''​SPLINE2''​. Normally ''​SPLINE2''​ is a good choice, but for high cutoffs (600 Ry for water) ''​SPLINE3''​ is better. The default (''​PW''​) is not bad, but generally inferior to the others.