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faq:ngrids [2016/03/30 10:20] (current)
oschuett crated from Juerg Hutter'
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 +====== How to choose the NGRIDS in &MGRID ======
 +
 +The three inputs "​CUTOFF",​ "​NGRIDS",​ and "​REL_CUTOFF"​ are important
 +for accuracy and efficiency. The problem is that both, "​accuracy and
 +efficiency"​ depend on the combination of chosen values. See also the [[faq:​cutoff|FAQ about cutoffs]].
 +
 +In an optimal setting you can proceed the following way:
 +
 +1) choose a high enough value for REL_CUTOFF
 +   this is system and basis set independent. It determines the
 +   ​minimal cutoff used to bring a Gaussian function on the real space grid.
 +   High values mean high accuracy, low values faster execution.
 +   
 +2) choose your cutoff and number of grids. This will determine the real space grids
 +   for your calulcation,​ e.g. cutoff=400 ngrids=4
 +   grid1 = 400, grid2 = 200, grid3 =100, grid4 = 50 (not actual numbers)
 +   The cutoff should be high enough to take on the Gaussian with the largest exponent,
 +   i.e. exponent*REL_CUTOFF <= cutoff (where exponent is for a product Gaussian)
 +   If you choose more grids the mapping (Gaussian->​Grid) is better but you have more
 +   ​overhead from more grids (mostly FFTs).
  
faq/ngrids.txt ยท Last modified: 2016/03/30 10:20 by oschuett