<div dir="ltr">Dear All, I did some testing on the size of simulation cell on my benzene-water cluster. Actually, my system is not a periodic system, so my setting is PERIODIC NONE in both &POISSON and &CELL sections. The structure of my benzene-water cluster was optimized by Gaussian09 and can be put into a cell with size as 5*5*5. I tried 7*7*7, 10*10*10, 15*15*15 and 20*20*20 cells for NVE calculation in CP2K with PBE functional. However, only cell with 20*20*20 can maintain the structure of benzene-water cluster. For the rest of results with smaller cell, all the structures break due to the dramatic increasing of temperature from 50 K to 10000 K after 30 steps. I have no idea about these testing results. Why could not use smaller cell? I consulted the CP2K reference and found a description on &MT section. Sets up parameters of Martyna-Tuckerman poisson solver. Note that exact results are only guaranteed if the unit cell is twice as large as charge density (and serious artefacts can result if the cell is much smaller). In addition, I noticed that the REL_CUTOFF value is related to the &MGRID section. Specify the multiplicative factor for the CUTOFF keyword in MULTI_GRID section. The result gives the cutoff at which the 1/r non-periodic FFT3D is evaluated.Default is 2.0 However, both of these lines do not have any detail explanations or examples. The CP2K reference is really too poor for a beginner to learn how to use CP2K, comparing the Gaussian reference. I do not understand what is the meaning of "if the cell is twice as large as charge density". How to check the charge density of my system? And also, is there any rule of thumb to choose the value of REL_CUTOFF? Best regards, Huan </div>