IMPORTANT NOTE: Using a barostat coupled to tilt dimensions xy, xz, yz can sometimes result in arbitrarily large values of the tilt dimensions, i.e. a dramatically deformed simulation box. LAMMPS -imposes reasonable limits on how large the tilt values can be, and -exits with an error if these are exceeded. This error typically +allows the tilt factors to grow a little beyond the normal limit +of half the box length (0.6 times the box length), and then performs +flipping or re-shaping to an equivalent periodic cell. The re-shaping +operation is described in more detail in the doc page for +fix deform. Both the barostat dynamics and +the atom trajectories are unaffected by this operation. However, +if a tilt factor is incremented by a large amount (1.5 times the +box length) on a single timestep, LAMMPS can not accomodate +this event and will terminate the simulation +with an error. This error typically indicates that there is something badly wrong with how the simulation -was constructed. The three most common sources of this error are -using keyword tri on a liquid system, specifying an external shear -stress tensor that exceeds the yield stress of the solid, and -specifying values of Pstart that are too far from the current stress -value. In other words, triclinic barostatting should be used with -care. This also is true for other barostat styles, although they tend -to be more forgiving of insults. +was constructed, such as specifying values of Pstart that are +too far from the current stress value, or specifying a timestep that +is too large. Triclinic barostatting should be used with +care. This also is true for other barostat styles, although they tend +to be more forgiving of insults. In particular, it is important to +recognize that equilibrium liquids can not support a shear stress and +that equilibrium solids can not support shear stresses that exceed the yield stress.
IMPORTANT NOTE: Unlike the fix temp/berendsen command which performs diff --git a/doc/fix_nh.txt b/doc/fix_nh.txt index 0ab1e76a19..a372e1f5fa 100644 --- a/doc/fix_nh.txt +++ b/doc/fix_nh.txt @@ -280,16 +280,24 @@ situations. IMPORTANT NOTE: Using a barostat coupled to tilt dimensions {xy}, {xz}, {yz} can sometimes result in arbitrarily large values of the tilt dimensions, i.e. a dramatically deformed simulation box. LAMMPS -imposes reasonable limits on how large the tilt values can be, and -exits with an error if these are exceeded. This error typically +allows the tilt factors to grow a little beyond the normal limit +of half the box length (0.6 times the box length), and then performs +flipping or re-shaping to an equivalent periodic cell. The re-shaping +operation is described in more detail in the doc page for +"fix deform"_fix_deform.html. Both the barostat dynamics and +the atom trajectories are unaffected by this operation. However, +if a tilt factor is incremented by a large amount (1.5 times the +box length) on a single timestep, LAMMPS can not accomodate +this event and will terminate the simulation +with an error. This error typically indicates that there is something badly wrong with how the simulation -was constructed. The three most common sources of this error are -using keyword {tri} on a liquid system, specifying an external shear -stress tensor that exceeds the yield stress of the solid, and -specifying values of {Pstart} that are too far from the current stress -value. In other words, triclinic barostatting should be used with -care. This also is true for other barostat styles, although they tend -to be more forgiving of insults. +was constructed, such as specifying values of {Pstart} that are +too far from the current stress value, or specifying a timestep that +is too large. Triclinic barostatting should be used with +care. This also is true for other barostat styles, although they tend +to be more forgiving of insults. In particular, it is important to +recognize that equilibrium liquids can not support a shear stress and +that equilibrium solids can not support shear stresses that exceed the yield stress. IMPORTANT NOTE: Unlike the "fix temp/berendsen"_fix_temp_berendsen.html command which performs