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° | SimSpeed | - | Set simulation speed |
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| | | Command | | Argument | | Datatype | | Default | | Min | | Max | |
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| Format: | | SimSpeed | Type = Normal | Fast | |
STRING | | - | | - |
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| Python: | | SimSpeed(Type) | | ||||||||||
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| Menu: | | Simulation > Force field | | ||||||||||
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| Related: | | Longrange , Interactions, Cutoff , ForceField, ScaleForce | | ||||||||||
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| Required: | |  | |
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The SimSpeed
command sets the speed/accuracy trade-off for the simulation algorithms. Currently it affects the treatment of longrange electrostatics and the force switching function near the cutoff in
(Y)AMBER force fields.
If Longrange
electrostatics are activated, the SimSpeed
command adjusts the Particle Mesh Ewald (PME) parameters. The main determinants for the PME accuracy are the grid spacing,
the tolerance for the direct space sum, and the spline interpolation order. With
'SimSpeed Normal', the tolerance is 1e-5 and the splines are of 6th order. With
'SimSpeed Fast', the tolerance is 1e-4 and the splines are of 4th order. The grid spacing cannot be adjusted and is always
<1 A.
If Longrange
electrostatics are not activated, the SimSpeed setting affects the treatment of forces near the
cutoff distance. With 'SimSpeed Fast' the forces are simply truncated. With
'SimSpeed Normal', a switching function is applied so that the forces smoothly converge to zero at the
cutoff distance.
Example 1:
SimSpeed Normal
Choose a normal simulation speed. If PME is activated (Longrange), the tolerance for the direct space sum is
1e-5, and the grid interpolation is done with 6th order splines.
Example 2:
SimSpeed Fast
Choose a fast simulation speed, treating long range electrostatics with reduced accuracy.