Hi, all,
    I was wondering to modify Shift electrostatic summation to a new one. The 
origin Shift form in Gromacs is like 1/r-1/Rc (Rc is cut-off), I need to modify 
to erfc(alpha*r)/r-erfc(alpha*Rc)/Rc, and the original long-range correction of 
Shift in code shift_util.c is q^2/(2*Rc) ( something like that ), I need to 
q^2*(erfc(alpha*Rc)/(2*Rc)+alpha/sqrt(pi)). So question is:
How the force to Shift was calculated? Gromacs use the differentiation to 
energy, got the force formula, then calculated the force? Or just use the 
numerical differentiation to the energy and got it? If is the latter one, I do 
not need to modify the force calculation.
And also, the alpha for Ewald summation, is there an option or parameters to 
adjust this value?
Hi, Mark and Berk, if I want do modify shift electrostatic summation as I want, 
what other files or place do I need to modify, except shift_util.c and force.c?
Shuangxing Dai
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