Dear all: In most studies, we focus on the accuracy of potential energy. How about the force? Consider two systems:
1. Two LJ particles (sigma=0.33nm, epsilon=0.625kJ/mol): If the force is cutoff at 2.5*sigma, so the maximum error in force is at the cutoff length and equal to ~0.08 kJ/mol nm. 2. Two point charges in a 3*3*3.3nm box (charge separation: 0.5 nm, q1=e, q2=-e): I tried the following calculations: a. Ewald summation, rcutoff=1.0nm, ewald_tol=1e-5, FFT_spacing=0.1nm: This produces: f1=-f2=542.2 kJ/mol nm; b. PME, rcutoff=1.0 nm, ewald_tol=1e-5; 4th order, FFT_spacing=0.1nm, this produces: f1=-f2=543.9 kJ/mol nm; c. PME, rcutoff=1.0 nm, ewald_tol=1e-5; 4th order, FFT_spacing=0.11nm, this produces: f1=543.97 kJ/mol nm, f2=-544.96 kJ/mol nm; d. PME, rcutoff=1.0 nm, ewald_tol=1e-5; 4th order, FFT_spacing=0.12nm, this produces: f1=543.43 kJ/mol nm, f2=-545.49 kJ/mol nm; Clearly, for LJ particles, the maximum accuracy is ~0.08kJ/mol nm - which sounds small. But for electrostatics, the error is quite large: if we take the Ewald results to be the exact solution, then for choice d (which seems to be a popular choice among gmx-users), the error amounts to be ~ 2.0 kJ/mol nm. Is this error acceptable? Thanks for your time. Nick
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