Parameter File - pscf_pc

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The pscf_pc and pscf_pg programs for periodic systems use nearly identical parameter file formats, and so are discussed together here.

Example

An example of a parameter file for the program pscf_pc3D is shown below.

System{
  Mixture{
    nMonomer  2
    monomers[
              A   1.0
              B   1.0
    ]
    nPolymer  1
    Polymer{
      type    linear
      nBlock  2
      blocks[
              0  0.25
              1  0.75
      ]
      phi     1.0
    }
    ds   0.01
  }
  Interaction{
    chi(
          1   0   20.0
    )
  }
  Domain{
    mesh             32  32  32
    lattice          cubic
    groupName        I_m_-3_m
  }
  Iterator{
    epsilon      1e-10
    maxItr       100
    maxHist      50
    isFlexible   1
  }
}

File Format

The main System block of a parameter file for any of the pscf_pc and pscf_pg programs for has the same overall structure as that used by pscf_fd program, and is described by the same format skeleton:

System{
  Mixture{ ...  }
  Interaction{ ...  }
  Domain{ ...  }
  Iterator#*{ ... }
  Sweep#*{ ... }
}

As for pscf_fd, the Sweep block is optional, and Iterator and Sweep blocks (if present) are both selectable blocks. The purposes of these top-level blocks have all been discussed previously in the discussion of pscf_fd, and will not be repeated here. Formats of the Mixture and Interaction blocks are the same for all PSCF programs, and so have also been discussed previously. Below, we discuss the contents of the Domain, Iterator, and Sweep blocks.

Domain Block

The Domain block contains information required to describe both unit cell of the periodic structure, the regular mesh that is used to discretize fields within each unit cell, and the space group of the desired solution. The parameter file format is discussed below, as well as in separate page here.

An example of the format of this block is shown below:

Domain{
  mesh        32  32  32
  lattice     cubic
  groupName   I_m_-3_m
}

The unitCell, mesh and groupName parameter lines are discussed separately below.

mesh

The value of the variable "mesh" is a vector of integers that specifies the number of grid points along each direction of the uniform spatial mesh used to discretize fields within each unit cell of the crystal. The number of integers is equal to the dimensionality D of the periodic unit cell. The format is

mesh   N[0] .... N[D-1]

where N[i] is the number of grid points along axis number i, for i = 0 ,..., D-1.

The dimensions N[0], ... , N[D-1] of the spatial mesh are initialized to the values given in the parameter file and cannot be changed during the lifetime of a program, except by rerunning the program with a modified parameter file.

lattice

The value of the parameter "lattice" is given by string that specifies the desired crystal system type. The set of allowed values for this string depends on the dimensionality of space. There are 7 allowed values of the lattice string for a 3D structure, corresponding to the 7 types of 3D crystal system, with names given by the strings "cubic", "tetragonal", "orthorhombic", "hexagonal", "rhombohedral", "monoclinic", and "triclinic". The allowed values for a 2D system include "square", "rectangular", "hexagonal" "rhombic" and "oblique". There is only allowed lattice system for a 1D crystal, which is referred to here as "lamellar". A more detailed discussion of each of the possible lattice systems is given in a separate page that documents conventions for unit cells.

groupName

The value of the string parameter "spaceGroup" is an identifier for the crystallographic space group of the desired structure. Each of the allowed values of the groupName string corresponds to a modified version of the name of a space group, as listed in the international tables of crystallography. In the above example, the groupName string I_m_-3_m is a modified form of the name \(Im\overline{3}m\) for the space group for a BCC lattice. Identifiers for crystallographic space groups are discussed and listed here.

The groupName is used to look up and read a representation of a space group that is used to generate a symmetry-adapted Fourier basis in which every basis function is invariant under all the elements of the specified space group. The algorithm used to construct this basis requires knowledge of a set of initial values for the unit cell parameters. The basis is thus normally constructed immediately after the first time unit cell parameters are initialized, e.g., by reading the values given in a file that contains a description of a field.

Unit cell parameters

To completely describe the Bravais lattice for a crystal, one must assign values to a list of parameters that give the lengths of the edges of the unit cell (or the Bravais lattice vectors) and, in some cases, angles between some of these edges. The number of unit cell parameters required to describe a unit cell is different for different lattice system. For example, a 3D cubic crystal can be described using a single parameter that gives the length of any edge of a cubic unit cell. Three parameters are required to describe an orthorhombic unit cell, to specify lengths of three orthogonal Bravais basis vectors, while 6 parameters are required for a a triclinic cell to specify 3 independent unit cell lengths and 3 independent angles. An array of unit cell parameters for each unit cell type that lists the required parameters in some pre-specified order. Conventions for the meaning of the required parameters and the order in which they are listed are described for each unit cell type in the documentation for unit cells.

Note that values for unit cell parameters are not specified in the parameter file the pscf_pc programs. These parameters values are instead specified within the header of any file that describes a field using a format that can be read by a pscf_pc or pscf_pg program. Values for unit cell parameters are thus normally first assigned values during execution of the first command in the command file that causes the program to read a field from file. There also exists a command named SET_UNIT_CELL that user can invoke to manually set or reset the unit cell parameters at any point during the processing of the command file.

The unit cell parameters for a crystal may be changed multiple times during program execution.

Iterators

The default Iterator for the pscf_pc and pscf_pg programs uses an Anderson-Mixing (AM) iterator algorithm. This can be invoked using either the generic label "Iterator" or the specific label "AmIterator". The only other iterator currently available is an Anderson-Mixing iterator designed specifically for problems in which a polymer melt or mixture is confined to a thin film (see Thin Films ).

Descriptions of the parameter file formats for the available iterators can be found by following the links in the table below:

Class	Description
AmIterator	Anderson Mixing iterator for periodic structures (default)
AmIteratorFilm	Thin Film Anderson Mixing iterator. Uses the Anderson Mixing algorithm to solve the SCFT equations for a system that is confined to a thin film.

The iterator class is formally optional (i.e., the program will finish reading the parameter file if it is omitted), but it is needed to perform any SCFT calculation.

Sweep

The only sweep algorithm currently avaiable for use with pspc_pc is a general linear sweep algorithm. This can be invoked by including a block that starts either with the generic label Sweep or the specific label LinearSweep. The required parameter file format for a linear sweep has been described here.

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