Syntax:
-fix ID group-ID efield ex ey ez +fix ID group-ID efield ex ey ez keyword value ...-
energy value = v_name + v_name = variable with name that calculates the potential energy of each atom in the added E-field ++
Examples:
@@ -37,11 +49,13 @@ can be specified as an equal-style or atom-style variable, namely ex, ey, ez. If the value is a variable, it should be specified as v_name, where name is the variable name. In this case, the variable will be evaluated each timestep, and -its value used to determine the E-field component. For point-dipoles, -equal-style variables can be used, but atom-style variables are not -currently supported, since they imply a spatial gradient in the -electric field which means additional terms with gradients of the -field are required for the force and torque on dipoles. +its value used to determine the E-field component. + +For point-dipoles, equal-style variables can be used, but atom-style +variables are not currently supported, since they imply a spatial +gradient in the electric field which means additional terms with +gradients of the field are required for the force and torque on +dipoles.
Equal-style variables can specify formulas with various mathematical functions, and include thermo_style command @@ -53,28 +67,71 @@ variables but can also include per-atom values, such as atom coordinates. Thus it is easy to specify a spatially-dependent E-field with optional time-dependence as well.
+Adding a force or torque to atoms implies a change in their potential +energy as they move or rotate due to the applied E-field. +
+For dynamics via the "run" command, this energy can be optionally +added to the system's potential energy for thermodynamic output (see +below). For energy minimization via the "minimize" command, this +energy must be added to the system's potential energy to formulate a +self-consistent minimization problem (see below). +
+The energy keyword is not allowed if the added field is a constant +vector (ex,ey,ez), with all components defined as numeric constants +and not as variables. This is because LAMMPS can compute the energy +for each charged particle directly as E = -x dot qE = -q (x*ex + y*ey ++ z*ez), so that -Grad(E) = F. Similarly for point-dipole particles +the energy can be computed as E = -mu dot E = -(mux*ex + muy*ey + +muz*ez), +
+The energy keyword is optional if the added force is defined with +one or more variables, and if you are performing dynamics via the +run command. If the keyword is not used, LAMMPS will set +the energy to 0.0, which is typically fine for dynamics. +
+The energy keyword is required if the added force is defined with +one or more variables, and you are performing energy minimization via +the "minimize" command for charged particles. It is not required for +point-dipoles, but a warning is issued since the minimizer in LAMMPS +does not rotate dipoles, so you should not expect to be able to +minimize the orientation of dipoles in an applied electric field. +
+The energy keyword specifies the name of an atom-style +variable which is used to compute the energy of each +atom as function of its position. Like variables used for ex, ey, +ez, the energy variable is specified as v_name, where name is the +variable name. +
+Note that when the energy keyword is used during an energy +minimization, you must insure that the formula defined for the +atom-style variable is consistent with the force +variable formulas, i.e. that -Grad(E) = F. For example, if the force +due to the electric field were a spring-like F = kx, then the energy +formula should be E = -0.5kx^2. If you don't do this correctly, the +minimization will not converge properly. +
Restart, fix_modify, output, run start/stop, minimize info:
No information about this fix is written to binary restart files.
The fix_modify energy option is supported by this -fix to add the potential "energy" inferred by the added force to the -system's potential energy as part of thermodynamic -output. This is a fictitious quantity but is -needed so that the minimize command can include the -forces added by this fix in a consistent manner. I.e. there is a -decrease in potential energy when atoms move in the direction of the -added force. The potential energy contribution of dipoles in the -electric field is also included. +fix to add the potential "energy" inferred by the added force due to +the electric field to the system's potential energy as part of +thermodynamic output. This is a fictitious +quantity but is needed so that the minimize command +can include the forces added by this fix in a consistent manner. +I.e. there is a decrease in potential energy when atoms move in the +direction of the added force due to the electric field.
This fix computes a global scalar and a global 3-vector of forces, which can be accessed by various output commands. The scalar is the potential energy discussed above. The vector is the total force added to the group of atoms. The scalar and vector values calculated by this fix -are "extensive". The added torques for point-dipoles cannot currently -be output in this manner. +are "extensive".
No parameter of this fix can be used with the start/stop keywords of the run command. diff --git a/doc/fix_efield.txt b/doc/fix_efield.txt index 04d88a87b2..dccb04c737 100644 --- a/doc/fix_efield.txt +++ b/doc/fix_efield.txt @@ -10,12 +10,17 @@ fix efield command :h3 [Syntax:] -fix ID group-ID efield ex ey ez :pre +fix ID group-ID efield ex ey ez keyword value ... :pre -ID, group-ID are documented in "fix"_fix.html command -efield = style name of this fix command -ex,ey,ez = E-field component values (electric field units) -any of ex,ey,ez can be a variable (see below) :ul +ID, group-ID are documented in "fix"_fix.html command :ulb,l +efield = style name of this fix command :l +ex,ey,ez = E-field component values (electric field units) :l +any of ex,ey,ez can be a variable (see below) :l +zero or more keyword/value pairs may be appended to args :l +keyword = {{energy} :l + {energy} value = v_name + v_name = variable with name that calculates the potential energy of each atom in the added E-field :pre +:ule [Examples:] @@ -34,11 +39,13 @@ can be specified as an equal-style or atom-style "variable"_variable.html, namely {ex}, {ey}, {ez}. If the value is a variable, it should be specified as v_name, where name is the variable name. In this case, the variable will be evaluated each timestep, and -its value used to determine the E-field component. For point-dipoles, -equal-style variables can be used, but atom-style variables are not -currently supported, since they imply a spatial gradient in the -electric field which means additional terms with gradients of the -field are required for the force and torque on dipoles. +its value used to determine the E-field component. + +For point-dipoles, equal-style variables can be used, but atom-style +variables are not currently supported, since they imply a spatial +gradient in the electric field which means additional terms with +gradients of the field are required for the force and torque on +dipoles. Equal-style variables can specify formulas with various mathematical functions, and include "thermo_style"_thermo_style.html command @@ -50,28 +57,71 @@ variables but can also include per-atom values, such as atom coordinates. Thus it is easy to specify a spatially-dependent E-field with optional time-dependence as well. +:line + +Adding a force or torque to atoms implies a change in their potential +energy as they move or rotate due to the applied E-field. + +For dynamics via the "run" command, this energy can be optionally +added to the system's potential energy for thermodynamic output (see +below). For energy minimization via the "minimize" command, this +energy must be added to the system's potential energy to formulate a +self-consistent minimization problem (see below). + +The {energy} keyword is not allowed if the added field is a constant +vector (ex,ey,ez), with all components defined as numeric constants +and not as variables. This is because LAMMPS can compute the energy +for each charged particle directly as E = -x dot qE = -q (x*ex + y*ey ++ z*ez), so that -Grad(E) = F. Similarly for point-dipole particles +the energy can be computed as E = -mu dot E = -(mux*ex + muy*ey + +muz*ez), + +The {energy} keyword is optional if the added force is defined with +one or more variables, and if you are performing dynamics via the +"run"_run.html command. If the keyword is not used, LAMMPS will set +the energy to 0.0, which is typically fine for dynamics. + +The {energy} keyword is required if the added force is defined with +one or more variables, and you are performing energy minimization via +the "minimize" command for charged particles. It is not required for +point-dipoles, but a warning is issued since the minimizer in LAMMPS +does not rotate dipoles, so you should not expect to be able to +minimize the orientation of dipoles in an applied electric field. + +The {energy} keyword specifies the name of an atom-style +"variable"_variable.html which is used to compute the energy of each +atom as function of its position. Like variables used for {ex}, {ey}, +{ez}, the energy variable is specified as v_name, where name is the +variable name. + +Note that when the {energy} keyword is used during an energy +minimization, you must insure that the formula defined for the +atom-style "variable"_variable.html is consistent with the force +variable formulas, i.e. that -Grad(E) = F. For example, if the force +due to the electric field were a spring-like F = kx, then the energy +formula should be E = -0.5kx^2. If you don't do this correctly, the +minimization will not converge properly. + [Restart, fix_modify, output, run start/stop, minimize info:] No information about this fix is written to "binary restart files"_restart.html. The "fix_modify"_fix_modify.html {energy} option is supported by this -fix to add the potential "energy" inferred by the added force to the -system's potential energy as part of "thermodynamic -output"_thermo_style.html. This is a fictitious quantity but is -needed so that the "minimize"_minimize.html command can include the -forces added by this fix in a consistent manner. I.e. there is a -decrease in potential energy when atoms move in the direction of the -added force. The potential energy contribution of dipoles in the -electric field is also included. +fix to add the potential "energy" inferred by the added force due to +the electric field to the system's potential energy as part of +"thermodynamic output"_thermo_style.html. This is a fictitious +quantity but is needed so that the "minimize"_minimize.html command +can include the forces added by this fix in a consistent manner. +I.e. there is a decrease in potential energy when atoms move in the +direction of the added force due to the electric field. This fix computes a global scalar and a global 3-vector of forces, which can be accessed by various "output commands"_Section_howto.html#howto_15. The scalar is the potential energy discussed above. The vector is the total force added to the group of atoms. The scalar and vector values calculated by this fix -are "extensive". The added torques for point-dipoles cannot currently -be output in this manner. +are "extensive". No parameter of this fix can be used with the {start/stop} keywords of the "run"_run.html command.