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git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@10081 f3b2605a-c512-4ea7-a41b-209d697bcdaa
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@ -13,12 +13,24 @@
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID efield ex ey ez
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<PRE>fix ID group-ID efield ex ey ez keyword value ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>efield = style name of this fix command
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<LI>ex,ey,ez = E-field component values (electric field units)
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>efield = style name of this fix command
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<LI>ex,ey,ez = E-field component values (electric field units)
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<LI>any of ex,ey,ez can be a variable (see below)
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<LI>zero or more keyword/value pairs may be appended to args
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<LI>keyword = <I><I>energy</I>
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<PRE> <I>energy</I> value = v_name
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v_name = variable with name that calculates the potential energy of each atom in the added E-field
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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@ -37,11 +49,13 @@ can be specified as an equal-style or atom-style
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<A HREF = "variable.html">variable</A>, namely <I>ex</I>, <I>ey</I>, <I>ez</I>. If the value is a
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variable, it should be specified as v_name, where name is the variable
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name. In this case, the variable will be evaluated each timestep, and
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its value used to determine the E-field component. For point-dipoles,
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equal-style variables can be used, but atom-style variables are not
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currently supported, since they imply a spatial gradient in the
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electric field which means additional terms with gradients of the
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field are required for the force and torque on dipoles.
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its value used to determine the E-field component.
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</P>
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<P>For point-dipoles, equal-style variables can be used, but atom-style
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variables are not currently supported, since they imply a spatial
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gradient in the electric field which means additional terms with
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gradients of the field are required for the force and torque on
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dipoles.
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</P>
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<P>Equal-style variables can specify formulas with various mathematical
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functions, and include <A HREF = "thermo_style.html">thermo_style</A> command
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@ -53,28 +67,71 @@ variables but can also include per-atom values, such as atom
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coordinates. Thus it is easy to specify a spatially-dependent E-field
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with optional time-dependence as well.
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</P>
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<HR>
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<P>Adding a force or torque to atoms implies a change in their potential
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energy as they move or rotate due to the applied E-field.
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</P>
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<P>For dynamics via the "run" command, this energy can be optionally
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added to the system's potential energy for thermodynamic output (see
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below). For energy minimization via the "minimize" command, this
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energy must be added to the system's potential energy to formulate a
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self-consistent minimization problem (see below).
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</P>
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<P>The <I>energy</I> keyword is not allowed if the added field is a constant
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vector (ex,ey,ez), with all components defined as numeric constants
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and not as variables. This is because LAMMPS can compute the energy
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for each charged particle directly as E = -x dot qE = -q (x*ex + y*ey
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+ z*ez), so that -Grad(E) = F. Similarly for point-dipole particles
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the energy can be computed as E = -mu dot E = -(mux*ex + muy*ey +
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muz*ez),
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</P>
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<P>The <I>energy</I> keyword is optional if the added force is defined with
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one or more variables, and if you are performing dynamics via the
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<A HREF = "run.html">run</A> command. If the keyword is not used, LAMMPS will set
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the energy to 0.0, which is typically fine for dynamics.
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</P>
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<P>The <I>energy</I> keyword is required if the added force is defined with
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one or more variables, and you are performing energy minimization via
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the "minimize" command for charged particles. It is not required for
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point-dipoles, but a warning is issued since the minimizer in LAMMPS
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does not rotate dipoles, so you should not expect to be able to
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minimize the orientation of dipoles in an applied electric field.
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</P>
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<P>The <I>energy</I> keyword specifies the name of an atom-style
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<A HREF = "variable.html">variable</A> which is used to compute the energy of each
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atom as function of its position. Like variables used for <I>ex</I>, <I>ey</I>,
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<I>ez</I>, the energy variable is specified as v_name, where name is the
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variable name.
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</P>
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<P>Note that when the <I>energy</I> keyword is used during an energy
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minimization, you must insure that the formula defined for the
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atom-style <A HREF = "variable.html">variable</A> is consistent with the force
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variable formulas, i.e. that -Grad(E) = F. For example, if the force
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due to the electric field were a spring-like F = kx, then the energy
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formula should be E = -0.5kx^2. If you don't do this correctly, the
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minimization will not converge properly.
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</P>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>.
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</P>
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<P>The <A HREF = "fix_modify.html">fix_modify</A> <I>energy</I> option is supported by this
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fix to add the potential "energy" inferred by the added force to the
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system's potential energy as part of <A HREF = "thermo_style.html">thermodynamic
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output</A>. This is a fictitious quantity but is
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needed so that the <A HREF = "minimize.html">minimize</A> command can include the
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forces added by this fix in a consistent manner. I.e. there is a
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decrease in potential energy when atoms move in the direction of the
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added force. The potential energy contribution of dipoles in the
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electric field is also included.
|
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fix to add the potential "energy" inferred by the added force due to
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the electric field to the system's potential energy as part of
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<A HREF = "thermo_style.html">thermodynamic output</A>. This is a fictitious
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quantity but is needed so that the <A HREF = "minimize.html">minimize</A> command
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can include the forces added by this fix in a consistent manner.
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I.e. there is a decrease in potential energy when atoms move in the
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direction of the added force due to the electric field.
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</P>
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<P>This fix computes a global scalar and a global 3-vector of forces,
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which can be accessed by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. The scalar is the potential
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energy discussed above. The vector is the total force added to the
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group of atoms. The scalar and vector values calculated by this fix
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are "extensive". The added torques for point-dipoles cannot currently
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be output in this manner.
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are "extensive".
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command.
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|
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@ -10,12 +10,17 @@ fix efield command :h3
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[Syntax:]
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fix ID group-ID efield ex ey ez :pre
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fix ID group-ID efield ex ey ez keyword value ... :pre
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ID, group-ID are documented in "fix"_fix.html command
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efield = style name of this fix command
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ex,ey,ez = E-field component values (electric field units)
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any of ex,ey,ez can be a variable (see below) :ul
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ID, group-ID are documented in "fix"_fix.html command :ulb,l
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efield = style name of this fix command :l
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ex,ey,ez = E-field component values (electric field units) :l
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any of ex,ey,ez can be a variable (see below) :l
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zero or more keyword/value pairs may be appended to args :l
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keyword = {{energy} :l
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{energy} value = v_name
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v_name = variable with name that calculates the potential energy of each atom in the added E-field :pre
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:ule
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[Examples:]
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@ -34,11 +39,13 @@ can be specified as an equal-style or atom-style
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"variable"_variable.html, namely {ex}, {ey}, {ez}. If the value is a
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variable, it should be specified as v_name, where name is the variable
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name. In this case, the variable will be evaluated each timestep, and
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its value used to determine the E-field component. For point-dipoles,
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equal-style variables can be used, but atom-style variables are not
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currently supported, since they imply a spatial gradient in the
|
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electric field which means additional terms with gradients of the
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field are required for the force and torque on dipoles.
|
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its value used to determine the E-field component.
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For point-dipoles, equal-style variables can be used, but atom-style
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variables are not currently supported, since they imply a spatial
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gradient in the electric field which means additional terms with
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gradients of the field are required for the force and torque on
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dipoles.
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Equal-style variables can specify formulas with various mathematical
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functions, and include "thermo_style"_thermo_style.html command
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@ -50,28 +57,71 @@ variables but can also include per-atom values, such as atom
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coordinates. Thus it is easy to specify a spatially-dependent E-field
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with optional time-dependence as well.
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:line
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Adding a force or torque to atoms implies a change in their potential
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energy as they move or rotate due to the applied E-field.
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|
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For dynamics via the "run" command, this energy can be optionally
|
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added to the system's potential energy for thermodynamic output (see
|
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below). For energy minimization via the "minimize" command, this
|
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energy must be added to the system's potential energy to formulate a
|
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self-consistent minimization problem (see below).
|
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|
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The {energy} keyword is not allowed if the added field is a constant
|
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vector (ex,ey,ez), with all components defined as numeric constants
|
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and not as variables. This is because LAMMPS can compute the energy
|
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for each charged particle directly as E = -x dot qE = -q (x*ex + y*ey
|
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+ z*ez), so that -Grad(E) = F. Similarly for point-dipole particles
|
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the energy can be computed as E = -mu dot E = -(mux*ex + muy*ey +
|
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muz*ez),
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The {energy} keyword is optional if the added force is defined with
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one or more variables, and if you are performing dynamics via the
|
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"run"_run.html command. If the keyword is not used, LAMMPS will set
|
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the energy to 0.0, which is typically fine for dynamics.
|
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|
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The {energy} keyword is required if the added force is defined with
|
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one or more variables, and you are performing energy minimization via
|
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the "minimize" command for charged particles. It is not required for
|
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point-dipoles, but a warning is issued since the minimizer in LAMMPS
|
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does not rotate dipoles, so you should not expect to be able to
|
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minimize the orientation of dipoles in an applied electric field.
|
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The {energy} keyword specifies the name of an atom-style
|
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"variable"_variable.html which is used to compute the energy of each
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atom as function of its position. Like variables used for {ex}, {ey},
|
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{ez}, the energy variable is specified as v_name, where name is the
|
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variable name.
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Note that when the {energy} keyword is used during an energy
|
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minimization, you must insure that the formula defined for the
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atom-style "variable"_variable.html is consistent with the force
|
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variable formulas, i.e. that -Grad(E) = F. For example, if the force
|
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due to the electric field were a spring-like F = kx, then the energy
|
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formula should be E = -0.5kx^2. If you don't do this correctly, the
|
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minimization will not converge properly.
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[Restart, fix_modify, output, run start/stop, minimize info:]
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No information about this fix is written to "binary restart
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files"_restart.html.
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The "fix_modify"_fix_modify.html {energy} option is supported by this
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fix to add the potential "energy" inferred by the added force to the
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system's potential energy as part of "thermodynamic
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output"_thermo_style.html. This is a fictitious quantity but is
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needed so that the "minimize"_minimize.html command can include the
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forces added by this fix in a consistent manner. I.e. there is a
|
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decrease in potential energy when atoms move in the direction of the
|
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added force. The potential energy contribution of dipoles in the
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electric field is also included.
|
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fix to add the potential "energy" inferred by the added force due to
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the electric field to the system's potential energy as part of
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"thermodynamic output"_thermo_style.html. This is a fictitious
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quantity but is needed so that the "minimize"_minimize.html command
|
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can include the forces added by this fix in a consistent manner.
|
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I.e. there is a decrease in potential energy when atoms move in the
|
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direction of the added force due to the electric field.
|
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|
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This fix computes a global scalar and a global 3-vector of forces,
|
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which can be accessed by various "output
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commands"_Section_howto.html#howto_15. The scalar is the potential
|
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energy discussed above. The vector is the total force added to the
|
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group of atoms. The scalar and vector values calculated by this fix
|
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are "extensive". The added torques for point-dipoles cannot currently
|
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be output in this manner.
|
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are "extensive".
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No parameter of this fix can be used with the {start/stop} keywords of
|
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the "run"_run.html command.
|
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|
|
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|||
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