[gmx-users] Shift Electrostatic Summation

[Shuangxing Dai]

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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