Hi,
I suggest those setting for AMBER force fields
constraints = h-bonds ; bonds involving H are constrained
rcoulomb = 1.0 ; short-range electrostatic cutoff (in nm)
rvdw = 1.0 ; short-range van der Waals cutoff (in nm)
vdw-modifier = Potential-shift-Verlet ; Amber specific
DispCorr = EnerPres ; account for cut-off vdW scheme
coulombtype = PME ; Particle Mesh Ewald for long-range electrostatics
fourierspacing = 0.125 ; grid spacing for FFT
best regards
Alessandra