ionic solution, restrictive primitive model (RPM)
compute s, (phi[osmotic coefficient]-1) and ln(gamma) from HARD SPHERE interaction(from Standard class)
2nd ~ 5th order integrals are included
p (<==> HSB) ==> integral = HSB* n! / (1-n)
s[n] = c^n * HSB[n] / (1-n)
ds[n]/dc = n * c ^ (n-1) * HSB[n] / (1-n) = s[n] * n/c cuz integral is independent of concentration
in simulation units
contains "linear step in sqrt(c)" && "linear step in c"