fix eos/table command

Syntax

fix ID group-ID eos/table style file N keyword
  • ID, group-ID are documented in fix command
  • eos/table = style name of this fix command
  • style = linear = method of interpolation
  • file = filename containing the tabulated equation of state
  • N = use N values in linear tables
  • keyword = name of table keyword corresponding to table file

Examples

fix 1 all eos/table linear eos.table 100000 KEYWORD

Description

Fix eos/table applies a tabulated mesoparticle equation of state to relate the particle internal energy (u_i) to the particle internal temperature (dpdTheta_i).

Fix eos/table creates interpolation tables of length N from internal energy values listed in a file as a function of internal temperature.

The interpolation tables are created by fitting cubic splines to the file values and interpolating energy values at each of N internal temperatures, and vice-versa. During a simulation, these tables are used to interpolate internal energy or temperature values as needed. The interpolation is done with the linear style.

For the linear style, the internal temperature is used to find 2 surrounding table values from which an internal energy is computed by linear interpolation, and vice-versa.

The filename specifies a file containing tabulated internal temperature and internal energy values. The keyword specifies a section of the file. The format of this file is described below.


The format of a tabulated file is as follows (without the parenthesized comments):

# EOS TABLE                (one or more comment or blank lines)

KEYWORD                    (keyword is first text on line)
N 500                      (N  parameter)
                           (blank)
1   1.00 0.000             (index, internal temperature, internal energy)
2   1.02 0.001
...
500 10.0 0.500

A section begins with a non-blank line whose 1st character is not a “#”; blank lines or lines starting with “#” can be used as comments between sections. The first line begins with a keyword which identifies the section. The line can contain additional text, but the initial text must match the argument specified in the fix command.

The next line lists the number of table entries. The parameter “N” is required and its value is the number of table entries that follow. Note that this may be different than the N specified in the fix eos/table command. Let Ntable = N in the fix command, and Nfile = “N” in the tabulated file. What LAMMPS does is a preliminary interpolation by creating splines using the Nfile tabulated values as nodal points. It uses these to interpolate as needed to generate energy and temperature values at Ntable different points. The resulting tables of length Ntable are then used as described above, when computing energy and temperature relationships. This means that if you want the interpolation tables of length Ntable to match exactly what is in the tabulated file (with effectively no preliminary interpolation), you should set Ntable = Nfile.

Following a blank line, the next N lines list the tabulated values. On each line, the 1st value is the index from 1 to N, the 2nd value is the internal temperature (in temperature units), the 3rd value is the internal energy (in energy units).

Note that the internal temperature and internal energy values must increase from one line to the next.

Note that one file can contain many sections, each with a tabulated potential. LAMMPS reads the file section by section until it finds one that matches the specified keyword.


Restrictions

This command is part of the USER-DPD package. It is only enabled if LAMMPS was built with that package. See the Making LAMMPS section for more info.

This command also requires use of the atom_style dpd command.

The equation of state must be a monotonically increasing function.

An error will occur if the internal temperature or internal energies are not within the table cutoffs.