faq:ngrids

# Differences

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 — faq:ngrids [2016/03/30 10:20] (current)oschuett crated from Juerg Hutter' 2016/03/30 10:20 oschuett crated from Juerg Hutter' 2016/03/30 10:20 oschuett crated from Juerg Hutter' Line 1: Line 1: + ====== 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).