Keywords: user

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Subsections

- 6.1 Keywords on the `GEOMETRY` directive
- 6.2 `SYMMETRY` -- Symmetry Group Input
- 6.3 Cartesian coordinate input
- 6.4 `ZMATRIX` -- Z-matrix input
- 6.5 `ZCOORD` -- Forcing internal coordinates
- 6.6 Applying constraints in geometry optimizations
- 6.7 `SYSTEM` -- Lattice parameters for periodic systems

# 6. Geometries

The `GEOMETRY` directive is a compound directive that allows the user to define the geometry to be used for a given calculation. The directive allows the user to specify the geometry with a relatively small amount of input, but there are a large number of optional keywords and additional subordinate directives that the user can specify, if needed. The directive therefore appears to be rather long and complicated when presented in its general form, as follows:

      GEOMETRY [<string name default geometry>] \
               [units <string units default angstroms>] \
               [(angstrom_to_au || ang2au) \
                      <real angstrom_to_au default 1.8897265>] \
               [print [xyz] || noprint] \
               [center || nocenter] \
               [bqbq] \
               [autosym [real tol default 1d-2]] \
               [autoz || noautoz] \
               [adjust] \
               [(nuc || nucl || nucleus) <string nucmodel>]

        [SYMMETRY [group] <string group_name> [print] \
               [tol <real tol default 1d-2>]]

        <string tag> <real x y z> [vx vy vz] [charge <real charge>] \
               [mass <real mass>] \
               [(nuc || nucl || nucleus) <string nucmodel>]
        ... ]

        [ZMATRIX || ZMT || ZMAT
             <string tagn> <list_of_zmatrix_variables>
             ...

             [VARIABLES
                  <string symbol> <real value>
                  ... ]

             [CONSTANTS
                  <string symbol> <real value>
                  ... ]

         (END || ZEND)]

             [ZCOORD
                  CVR_SCALING <real value>
                  BOND    <integer i> <integer j> \
                          [<real value>] [<string name>] [constant]
                  ANGLE   <integer i> <integer j> \
                          [<real value>] [<string name>] [constant]
                  TORSION <integer i> <integer j> <integer k> <integer l> \
                          [<real value>] [<string name>] [constant]
              END]

              [SYSTEM surface  <molecule polymer surface crystal default molecule>
                   lat_a <real lat_a> lat_b <real lat_b> lat_c <real lat_c>
                   alpha <real alpha> beta <real beta> gamma <real gamma>
              END]

       END

The three main parts of the `GEOMETRY` directive are:

- keywords on the first line of the directive (to specify such optional input as the geometry name, input units, and print level for the output)
- symmetry information
- Cartesian coordinates or Z-matrix input to specify the locations of the atoms and centers
- lattice parameters (needed only for periodic systems)

The following sections present the input for this compound directive in detail, describing the options available and the usages of the various keywords in each of the three main parts.

#
6.1 Keywords on the `GEOMETRY` directive

This section presents the options that can be specified using the keywords and optional input on the main line of the `GEOMETRY` directive. As described above, the first line of the directive has the general form,

      GEOMETRY [<string name default geometry>] \
               [units <string units default angstroms>] \
               [bqbq] \
               [print [xyz] || noprint] \
               [center || nocenter] \
               [autosym [real tol default 1d-2]] \
               [autoz || noautoz] \
               [adjust] \
               [(nuc || nucl || nucleus) <string nucmodel>]

All of the keywords and input on this line are optional. The following list describes all options and their defaults.

- `<name>` - user-supplied name for the geometry; the default name is `geometry`, and all NWChem modules look for a geometry with this name. However, multiple geometries may be specified by using a different name for each. Subsequently, the user can direct a module to a named geometry by using the `SET` directive (see the example in Section 5.7) to associate the default name of `geometry` with the alternate name.

- `units` - keyword specifying that a value will be entered by the user for the string variable `<units>`. The default units for the geometry input are Angstrøms (Note: atomic units or Bohr are used within the code, regardless of the option specified for the input units. The default conversion factor used in the code to convert from Angstrøms to Bohr is which may be overidden with the `angstrom_to_au` keyword described below.). The code recognizes the following possible values for the string variable `<units>`:
  - `angstroms` or `an` -- Angstroms (Å), the default (converts to A.U. using the Åto A.U. conversion factor)
  - `au` or `atomic` or `bohr` -- Atomic units (A.U.)
  - `nm` or `nanometers` -- nanometers (converts to A.U. using a conversion factor computed as times the Å to A.U. conversion factor)
  - `pm` or `picometers` -- picometers (converts to A.U. using a conversion factor computed as times the Å to A.U. conversion factor)

- `angstrom_to_au` - may also be specified as `ang2au`. This enables the user to modify the conversion factors used to convert between Å and A.U.. The default value is .

- `bqbq` - keyword to specify the treatment of interactions between dummy centers. The default in NWChem is to ignore such interactions when computing energies or energy derivatives. These interactions will be included if the keyword `bqbq` is specified.

- `print` and `noprint` - complementary keyword pair to enable or disable printing of the geometry. The default is to print the output associated with the geometry. In addition, the keyword `print` may be qualified by the additional keyword `xyz`, which specifies that the coordinates should be printed in the XYZ format of molecular graphics program XMol.

- `center` and `nocenter` - complementary keyword pair to enable or disable translation of the center of nuclear charge to the origin. With the origin at this position, all three components of the nuclear dipole are zero. The default is to move the center of nuclear charge to the origin.

- `autosym` - keyword to specify that the symmetry of the geometric system should be automatically determined. This option is on by default. Only groups up to and including are recognized. Occasionally NWChem will be unable to determine the full symmetry of a molecular system, but will find a proper subgroup of the full symmetry. The default tolerance is set to work for most cases, but may need to be decreased to find the full symmetry of a geometry. Note that autosym will be turned off if the `SYMMETRY` group input is given (See section 6.2).

- `noautoz` - by default NWChem (release 3.3 and later) will generate redundant internal coordinates from user input Cartesian coordinates. The internal coordinates will be used in geometry optimizations. The `noautoz` keyword disables use of internal coordinates. The `autoz` keyword is provided only for backward compatibility. See Section 6.5 for a more detailed description of redundant internal coordinates, including how to force the definition of specific internal variables in combination with automatically generated variables.

- `adjust` - This indicates that an existing geometry is to be adjusted. Only new input for the redundant internal coordinates may be provided (Section 6.5). It is not possible to define new centers or to modify the point group using this keyword. See Section 6.5 for an example of its usage.

- `nucleus` - keyword to specify the default model for the nuclear charge distribution. The following values are recognized:
  - `point` or `pt` -- point nuclear charge distribution. This is the default.
  - `finite` or `fi` -- finite nuclear charge distribution with a Gaussian shape. The RMS radius of the Gaussian is determined from the nuclear mass number by the expression fm.
  NOTE: If you specify a finite nuclear size, you should ensure that the basis set you use is contracted for a finite nuclear size. See the Section 7 for more information.

The following examples illustrate some of the various options that the user can specify on the first input line of the `GEOMETRY` directive, using the keywords and input options described above.

The following directives all specify the same geometry for (a bond length of 0.732556 Å):

      geometry                           geometry units nm
        h 0 0 0                            h 0 0 0
        h 0 0 0.732556                     h 0 0 0.0732556
      end                                end

      geometry units pm                  geometry units atomic
        h 0 0 0                            h 0 0 0
        h 0 0 73.2556                      h 0 0 1.3843305
      end                                end

#
6.2 `SYMMETRY` -- Symmetry Group Input

The `SYMMETRY` directive is used (optionally) within the compound `GEOMETRY` directive to specify the point group for the molecular geometry. The general form of the directive, as described above within the general form of the `GEOMETRY` directive, is as follows:

        [SYMMETRY [group] <string group_name> [print] \
               [tol <real tol default 1d-2>]]

The keyword `group` is optional, and can be omitted without affecting how the input for this directive is processed6.1. However, if the `SYMMETRY` directive is used, a group name must be specified by supplying an entry for the string variable `<group_name>`. The group name should be specified as the standard Schöflies symbol. Examples of expected input for the variable `group_name` include such entries as:

- `c2v` - for molecular symmetry
- `d2h` - for molecular symmetry
- `Td` - for molecular symmetry
- `d6h` - for molecular symmetry

The `SYMMETRY` directive is optional. The default is no symmetry (i.e., point group). Automatic detection of point group symmetry is available through the use of `autosym` in the `GEOMETRY` directive main line (discussed in Section 6.1). Note: if the `SYMMETRY` directive is present the `autosym` keyword is ignored.

If only symmetry-unique atoms are specified, the others will be generated through the action of the point group operators, but the user if free to specify all atoms. The user must know the symmetry of the molecule being modeled, and be able to specify the coordinates of the atoms in a suitable orientation relative to the rotation axes and planes of symmetry. Appendix C lists a number of examples of the `GEOMETRY` directive input for specific molecules having symmetry patterns recognized by NWChem. The exact point group symmetry will be forced upon the molecule, and atoms within A.U. of a symmetry element (e.g., a mirror plane or rotation axis) will be forced onto that element. Thus, it is not necessary to specify to a high precision those coordinates that are determined solely by symmetry.

The keyword `print` gives information concerning the point group generation, including the group generators, a character table, the mapping of centers, and the group operations.

The keyword `tol` relates to the accuracy with which the symmetry-unique atoms should be specified. When the atoms are generated, those that are within the tolerance, `tol`, are considered the same.

#
6.3 Cartesian coordinate input

The default in NWChem is to specify the geometry information entirely in Cartesian coordinates, and examples of this format have appeared above (e.g, Section 2.3). Each center (usually an atom) is identified on a line of the following form:

        <string tag> <real x y z> [vx vy vz] \
            [charge <real charge>] [mass <real mass>] \
            [(nuc || nucl || nucleus) <string nucmodel>]

The string `<tag>` is the name of the atom or center, and its case (upper or lower) is important. The tag is limited to 16 characters and is interpreted as follows:

- If the entry for `<tag>` begins with either the symbol or name of an element (regardless of case), then the center is treated as an atom of that type. The default charge is the atomic number (adjusted for the presence of ECPs by the ECP `NELEC` directive ; see Section 8). Additional characters can be added to the string, to distinguish between atoms of the same element (For example, the tags `oxygen`, `O`, `o34`, `olonepair`, and `Oxygen-ether`, will all be interpreted as oxygen atoms.).
- If the entry for `<tag>` begins with the characters `bq` or `x` (regardless of case), then the center is treated as a dummy center with a default zero charge (Note: a tag beginning with the characters `xe` will be interpreted as a xenon atom rather than as a dummy center.). Dummy centers may optionally have basis functions or non-zero charge. See Section B.2 for a sample input using dummy centers with charges.

It is important to be aware of the following points regarding the definitions and usage of the values specified for the variable `<tag>` to describe the centers in a system:

- If the tag begins with characters that cannot be matched against an atom, and those characters are not `BQ` or `X`, then a fatal error is generated.
- The tag of a center is used in the `BASIS` (Section 7) and `ECP` (Section 8) directives to associate functions with centers.
- All centers with the same tag will have the same basis functions.
- When using automatic symmetry detection, only centers with the same tag will be candidates for testing for symmetry equivalence.
- The user-specified charges (of all centers, atomic and dummy) and any net total charge of the system (Section 5.11) are used to determine the number of electrons in the system.

The Cartesian coordinates of the atom in the molecule are specified as real numbers supplied for the variables `x`, `y`, and `z` following the characters entered for the tag. The values supplied for the coordinates must be in the units specified by the value of the variable `<units>` on the first line of the `GEOMETRY` directive input.

After the Cartesian coordinate input, optional velocities may be entered as real numbers for the variables `vx`, `vy`, and `vz`. The velocities should be given in atomic units and are used in QMD and PSPW calculations.

The Cartesian coordinate input line also contains the optional keywords `charge`, `mass` and `nucleus`, which allow the user to specify the charge of the atom (or center) and its mass (in atomic mass units), and the nuclear model. The default charge for an atom is its atomic number, adjusted for the presence of ECPs (see Section 8). In order to specify a different value for the charge on a particular atom, the user must enter the keyword `charge`, followed by the desired value for the variable `<charge>`.

The default mass for an atom is taken to be the mass of its most abundant naturally occurring isotope or of the isotope with the longest half-life. To model some other isotope of the element, its mass must be defined explicitly by specifying the keyword `mass`, followed by the value (in atomic mass units) for the variable `<mass>`.

The default nuclear model is a point nucleus. The keyword `nucleus` (or `nucl` or `nuc`) followed by the model name `<nucmodel>` overrides this default. Allowed values of `<nucmodel>` are `point` or `pt` and `finite` or `fi`. The `finite` option is a nuclear model with a Gaussian shape. The RMS radius of the Gaussian is determined by the atomic mass number via the formula fm. The mass number is derived from the variable `<mass>`.

The geometry of the system can be specified entirely in Cartesian coordinates by supplying a `<tag>` line of the type described above for each atom or center. The user has the option, however, of supplying the geometry of some or all of the atoms or centers using a Z-matrix description. In such a case, the user supplies the input tag line described above for any centers to be described by Cartesian coordinates, and then specifies the remainder of the system using the optional `ZMATRIX` directive described below in Section 6.4.

#
6.4 `ZMATRIX` -- Z-matrix input

The `ZMATRIX` directive is an optional directive that can be used within the compound `GEOMETRY` directive to specify the structure of the system with a Z-matrix, which can include both internal and Cartesian coordinates. The `ZMATRIX` directive is itself a compound directive that can include the `VARIABLES` and `CONSTANTS` directives, depending on the options selected. The general form of the compound `ZMATRIX` directive is as follows:

        [ZMATRIX || ZMT || ZMAT
             <string tagn> <list_of_zmatrix_variables>
             ...

             [VARIABLES
                  <string symbol> <real value>
                  ... ]

             [CONSTANTS
                  <string symbol> <real value>
                  ... ]

        (END || ZEND)]

The input module recognizes three possible spellings of this directive name. It can be invoked with `ZMATRIX`, `ZMT`, or `ZMAT`. The user can specify the molecular structure using either Cartesian coordinates or internal coordinates (bond lengths, bond angles and dihedral angles. The Z-matrix input for a center defines connectivity, bond length, and bond or torsion angles. Cartesian coordinate input for a center consists of three real numbers defining the x,y,z coordinates of the atom.

Within the Z-matrix input, bond lengths and Cartesian coordinates must be input in the user-specified units, as defined by the value specified for the variable `<units>` on the first line of the `GEOMETRY` directive. All angles are specified in degrees.

The individual centers (denoted as `i`, `j`, and `k` below) used to specify Z-matrix connectivity may be designated either as integers (identifying each center by number) or as tags (If tags are used, the tag must be unique for each center.) The use of ``dummy'' atoms is possible, by using `X` or `BQ` at the start of the tag.

Bond lengths, bond angles and dihedral angles (denoted below as `R`, `alpha`, and `beta`, respectively) may be specified either as numerical values or as symbolic strings that must be subsequently defined using the `VARIABLES` or `CONSTANTS` directives. The numerical values of the symbolic strings labeled `VARIABLES` may be subject to changes during a geometry optimization say, while the numerical values of the symbolic strings labeled `CONSTANTS` will stay frozen to the value given in the input. The same symbolic string can be used more than once, and any mixture of numeric data and symbols is acceptable. Bond angles () must be in the range .

The Z-matrix input is specified sequentially as follows:

       tag1
       tag2 i R
       tag3 i R j alpha
       tag4 i R j alpha k beta [orient]
       ...

The structure of this input is described in more detail below. In the following discussion, the tag or number of the center being currently defined is labeled as `C` (``C'' for current). The values entered for these tags for centers defined in the Z-matrix input are interpreted in the same way as the `<tag>` entries for Cartesian coordinates described above (see Section 6.3). Figures 6.1, 6.2 and 6.3 display the relationships between the input data and the definitions of centers and angles.

Figure 6.1: Relationships between the centers, bond angle and dihedral angle in Z-matrix input.

Figure 6.2: Relationships between the centers and two bond angles in Z-matrix input with optional parameter specified as .

Figure 6.3: Relationships between the centers and two bond angles in Z-matrix input with optional parameter specified as .

The Z-matrix input shown above is interpreted as follows:

1. `tag1`

   Only a tag is required for the first center.

2. `tag2 i R`

   The second center requires specification of its tag and the bond length () distance to a previous atom, which is identified by `i`.

3. `tag3 i R j alpha`

   The third center requires specification of its tag, its bond length distance () to one of the two previous centers (identified by the value of `i`), and the bond angle .

4. `tag i R j alpha k beta [<integer orient default 0>]`

   The fourth, and all subsequent centers, require the tag, a bond length () relative to center `i`, the bond angle with centers `i` and `j` ( ), and either

   1. the dihedral angle () between the current center and centers `i`, `j`, and `k` (Figure 6.1), or
   2. a second bond angle and an orientation to the plane containing the other three centers (Figure 6.2 and 6.3).

   By default, is interpreted as a dihedral angle (see Figure 6.1), but if the optional final parameter (`<orient>`) is specified with the value , then is interpreted as the angle . The sign of `<orient>` specifies the direction of the bond angle relative to the plane containing the three reference atoms. If `<orient>` is , then the new center (`C`) is above the plane (Figure 6.2); and if `<orient>` is , then `C` is below the plane (Figure 6.3).

Following the Z-matrix center definitions described above, the user can specify initial values for any symbolic variables used to define the Z-matrix tags. This is done using the optional `VARIABLES` directive, which has the general form:

      VARIABLES
        <string symbol>  <real value>
        ...

Each line contains the name of a variable followed by its value. Optionally, an equals sign (`=`) can be included between the symbol and its value, for clarity in reading the input file.

Following the `VARIABLES` directive, the `CONSTANTS` directive may be used to define any Z-matrix symbolic variables that remain unchanged during geometry optimizations. To freeze the Cartesian coordinates of an atom, refer to Section 6.6. The general form of this directive is as follows:

      CONSTANTS
        <string symbol>  <real value>
        ...

Each line contains the name of a variable followed by its value. As with the `VARIABLES` directive, an equals sign (`=`) can be included between the symbol and its value.

The end of the Z-matrix input using the compound `ZMATRIX` directive is signaled by a line containing either `END` or `ZEND`, following all input for the directive itself and its associated optional directives.

A simple example is presented for water. All Z-matrix parameters are specified numerically, and symbolic tags are used to specify connectivity information. This requires that all tags be unique, and therefore different tags are used for the two hydrogen atoms, which may or may not be identical.

      geometry
        zmatrix
          O
          H1 O 0.95
          H2 O 0.95 H1 108.0
        end
      end

The following example illustrates the Z-matrix input for the molecule . This input uses the numbers of centers to specify the connectivity information (`i`, `j`, and `k`), and uses symbolic variables for the Z-matrix parameters `R`, `alpha`, and `beta`, which are defined in the inputs for the `VARIABLES` and `CONSTANTS` directives.

    geometry
     zmatrix
       C
       C 1 CC
       H 1 CH1 2 HCH1
       H 1 CH2 2 HCH2 3  TOR1  0
       H 1 CH3 2 HCH3 3 -TOR2  0
       F 2 CF1 1 CCF1 3  TOR3  0
       F 2 CF2 1 CCF2 6  FCH1  1
       F 2 CF3 1 CCF3 6  FCH2 -1
       variables
         CC    1.4888
         CH1   1.0790
         CH2   1.0789
         CH3   1.0789
         CF1   1.3667
         CF2   1.3669
         CF3   1.3669
       constants
         HCH1  104.28
         HCH2  104.74
         HCH3  104.7
         CCF1  112.0713
         CCF2  112.0341
         CCF3  112.0340
         TOR1  109.3996
         TOR2  109.3997
         TOR3  180.0000
         FCH1  106.7846
         FCH2  106.7842
     end
    end

The input for any centers specified with Cartesian coordinates must be specified using the format of the `<tag>` lines described in Section 6.3 above. However, in order to correctly specify these Cartesian coordinates within the Z-matrix, the user must understand the orientation of centers specified using internal coordinates. These are arranged as follows:

- The first center is placed at the origin.
- The second center is placed along the positive z-axis.
- The third center is placed in the z-x plane.

#
6.5 `ZCOORD` -- Forcing internal coordinates

By default redundant internal coordinates are generated for use in geometry optimizations. Connectivity is inferred by comparing inter-atomic distances with the sum of the van der Waals radii of the two atoms involved in a possible bond, times a scaling factor. The scaling factor is an input parameter of `ZCOORD` which maybe changed from its default value of 1.3. Under some circumstances (unusual bonding, bond dissociation, ...) it will be necessary to augment the automatically generated list of internal coordinates to force some specific internal coordinates to be included in among the internal coordinates. This is accomplished by including the optional directive `ZCOORD` within the geometry directive. The general form of the `ZCOORD` directive is as follows:

      ZCOORD
         CVR_SCALING <real value>
         BOND    <integer i> <integer j> \
                 [<real value>] [<string name>] [constant]
         ANGLE   <integer i> <integer j> <integer k> \
                 [<real value>] [<string name>] [constant]
         TORSION <integer i> <integer j> <integer k> <integer l> \
                 [<real value>] [<string name>] [constant]
      END

The centers `i`, `j`, `k` and `l` must be specified using the numbers of the centers, as supplied in the input for the Cartesian coordinates. The `ZCOORD` input parameters are defined as follows:

- `cvr_scaling` -- scaling factor applied to van der Waals radii.
- `bond` -- a bond between the two centers.
- `angle` -- a bond angle .
- `torsion` -- a torsion (or dihedral) angle. The angle between the planes `i-j-k` and `j-k-l`.

A value may be specified for a user-defined internal coordinate, in which case it is forced upon the input Cartesian coordinates while attempting to make only small changes in the other internal coordinates. If no value is provided the value implicit in the input coordinates is kept. If the keyword `constant` is specified, then that internal variable is not modified during a geometry optimization with DRIVER (Section 20). Each internal coordinate may also be named either for easy identification in the output, or for the application of constraints (Section 6.6).

If the keyword `adjust` is specified on the main `GEOMETRY` directive, only `ZCOORD` data may be specified and it can be used to change the user-defined internal coordinates, including adding/removing constraints and changing their values.

#
6.6 Applying constraints in geometry optimizations

Internal coordinates specified as constant in a `ZCOORD` directive or in the constants section of a `ZMATRIX` directive, will be frozen at their initial values if a geometry optimization is performed with DRIVER (Section 20).

If internal coordinates have the same name (give or take an optional sign for torsions) then they are forced to have the same value. This may be used to force bonds or angles to be equal even if they are not related by symmetry.

When atoms have been specified by their Cartesian coordinates, and internal coordinates are not being used, it is possible to freeze the cartesian position of selected atoms. This is useful for such purposes as optimizing a molecule absorbed on the surface of a cluster with fixed geometry. Only the gradients associated with the active atoms are computed. This can result in a big computational saving, since gradients associated with frozen atoms are forced to zero (Note, however, that this destroys the translational and rotational invariance of the gradient. This is not yet fully accommodated by the STEPPER geometry optimization software, and can sometimes result in slower convergence of the optimization. The DRIVER optimization package does not suffer from this problem).

The `SET` directive (Section 5.7) is used to freeze atoms, by specifying a directive of the form:

      set geometry:actlist <integer list_of_center_numbers>

This defines only the centers in the list as active. All other centers will have zero force assigned to them, and will remain frozen at their starting coordinates during a geometry optimization.

For example, the following directive specifies that atoms numbered 1, 5, 6, 7, 8, and 15 are active and all other atoms are frozen:

      set geometry:actlist 1 5:8 15

or equivalently,

      set geometry:actlist 1 5 6 7 8 15

If this option is not specified by entering a `SET` directive, the default behavior in the code is to treat all atoms as active. To revert to this default behavior after the option to define frozen atoms has been invoked, the `UNSET` directive must be used (since the database is persistent, see Section 3.2). The form of the `UNSET` directive is as follows:

      unset geometry:actlist

#
6.7 `SYSTEM` -- Lattice parameters for periodic systems

This keyword is needed only for for 1-, 2-, and 3-dimensional periodic systems.

The `system` keyword can assume the following values

- `polymer` -- system with 1-d translational symmetry.
- `surface` -- system with 2-d translational symmetry.
- `crystal` -- system with 3-d translational symmetry.
- `molecule` -- no translational symmetry (this is the default)

When the system possess translational symmetry, fractional coordinates are used in the directions where translational symmetry exists. This means that for crystals , and are fractional, for surfaces and are fractional, whereas for polymers only is fractional. For example, in the following HO layer input (a 2-d periodic system), and coordinates are fractional, whereas is expressed in Å.

    geometry units angstrom
      O     0.353553    0.353553         2.100000000
      H     0.263094    0.353553         2.663590000
      H     0.444007    0.353553         2.663590000

Since no space group symmetry is available yet other than , input of cell parameters is relative to the primitive cell. For example, this is the input required for the cubic face-centered type structure of bulk MgO.

      system crystal
       lat_a 2.97692 lat_b 2.97692 lat_c 2.97692
       alpha 60.00 beta 60.00 gamma 60.00
      end

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