Hi everyone,
   I am doing APBS calculations on a protein system with two conformations. I would like to get the electrostatic energy difference between the two conformations. However, when I fixed the box size and changed the grid size (by changing the number of grid points along each axis), the energy differences seem to be too big.
   I set the center of the grid to be the center of the molecule in both conformations. I used exactly the same input parameters except changing dime. The energy I obtained got bigger when the grid is getting finer.
   Can anyone tell me the possible reason? or is there any mistakes in my input file? Here is the input parameters:

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read
  mol pqr ack_last.pqr
end
elec name first
  mg-auto
  dime 353 481 289
  cglen 186.0 213.0 134.0
  cgcent mol 1
  fglen 146 173 95
  fgcent mol 1
  mol 1
  lpbe
  bcfl sdh
  srfm smol
  chgm spl2
  ion 1 0.150 0.95
  ion -1 0.150 1.81
  pdie  2.0
  sdie  78.54
  sdens  10.0
  srad  1.4
  swin  0.3
  temp  298.15
  gamma  0.105
  calcenergy total
  calcforce no
  write pot dx pot
end
print energy first end
quit

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Thanks,

Cheers,
Haiguang