I did an MD simulation of 1 NA+, 1 K+ and 2 CL- ions in water. Water box; L= 24.83A, 467 water molecules, cutoff = L/2. When computing the second moment of acceleration for NA+ as; <U^2> = <F^2>/M^2, I find that <U^2> in x,y and z were all very large values, perhaps due to the large force<F^2> divided by a very small mass:
<Ux^2>: 4.6407371750376985e+55
<Uy^2>: 1.0500429357960323e+56
<Uz^2>: 8.992860163533591e+55
Average <U^2>: 8.044675565510538e+55
NA_MASS_KG = 22.989769 * 1.66053906660e-27 = 3.825 x 10^(-26) kg .
I am trying to replicate a result from a paper that has Average <U^2> = 2.21*10(^4). I followed the above parameters mentioned but was wondering if there is any other setting or step I need to check carefully that could be responsible for this large 2nd moment of acceleration.