[Apbs-users] a question about solvation energies

[Assa S. <as...@wi...>]

Hi, i'm trying to calculate solvation energies of a protein in two
different conformations, and i've got a few questions, and excuse me if
they're stupid (today's the 3rd day i'm using apbs):
1. as i know it, and correct me if i'm wrong, i have to calculate , (for
each conformation in my case), the difference in total energy energy
between the "solvated protein" (using solvent dielectric) and the
protein in vacuo. 
i did it, using different grid spacing, and received interesting
results:
1. for two grids with the same size but different spacing, i have
performed a solvation calculation (=totEsolv-totEvacuo), i received
different absolute results for total energy(totEsolv and totEvacuo), but
the solvation energies (=totEsolv-totEvacuo)  stayed very close in both
grid resolutions. i understand that there are artifacts, due to the
discrete nature of the grid, but are they so strong? here are the
figures:
in both cases i used Grid lengths: 58.761 x 32.129 x 25.828 
in which the protein fits very comfortably.
for a 161 x 97 x 97 grid:
totEsolv =9.534619136962E+04 kJ/mol 
totEvacuo =9.875123989039E+04 kJ/mol
totEsolv-totEvacuo= -0.340504852 E+04 kJ/mol
for a 161 x 129 x 129 grid :
totEsolv = 6.803247450678E+04 kJ/mol
totEvacuo = 7.141364011794E+04 kJ/mol
totEsolv-totEvacuo=-0.338116561 E+04 kJ/mol
i read some posting about it, that said that the coulombic energy should
be added:
http://cholla.wustl.edu/pipermail/apbs-users/2003-October/000104.html
can anyone give me a little deeper explanation about why it's done, and
how can this correction be grid-resolution-dependant (since i need a
different correction for each grid obviously)?is there also an effect of
grid size (for a given spacing), because of the larget solvent volume?
2. what is a legitimate grid spacing (in angstroms) for calculations, in
order for the to be meaningful (but not necessarily very accurate)?
3. how can i estimate the error, in the energy calculations? (except for
running many calculations for different orientations on the molecule)
thanks a lot in advance,
T