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Keywords for the GEOMETRY directive

Keywords for 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] || noautosym]  
          [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

<name> is a 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 the SET Section) to associate the default name of geometry with the alternate name.

UNITS

The units keyword specifies what 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 1.8897265 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: Angstrøms , the default (converts to A.U. using the Angstrøm to 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 10.0 times the Angstrøm to A.U. conversion factor)
  • pm or picometers: picometers (converts to A.U. using a conversion factor computed as 0.01 times the Angstrøm to A.U. conversion factor)

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 H2 (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

ANGSTROM_TO_AU

The angstrom_to_au option may also be specified as ang2au. This enables the user to modify the conversion factors used to convert between Angstrøm and A.U.. The default value is 1.8897265.

BQBQ

The bqbq keyword specifies 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/NOPRINT

print and noprint is a 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/NOCENTER

center and nocenter is a 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/NOAUTOSYM

autosym and noautosym are keywords to specify that the symmetry of the geometric system should be automatically determined. This option is on by default, but can be turned off with noautosym. Only groups up to and including Oh 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 Symmetry Group Input). Also note that if symmetry equivalent atoms have different tags in the geometry they will not be detected as symmetry equivalent by the autosym capability. The reason for this is that atoms with different tags might be assigned different basis sets, for example, after which they are no longer symmetry equivalent. Therefore autosym chooses to make the save choice.

AUTOZ/NOAUTOZ

By default NWChem 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 Forcing internal coordinates 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

Use of the adjust keyword indicates that an existing geometry is to be adjusted. Only new input for the redundant internal coordinates may be provided (ZCOORD: Forcing internal coordinates). It is not possible to define new centers or to modify the point group using this keyword. See the section use of the adjust keyword for an example of its usage.

NUCLEUS

The nucleus keyword to specifies 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 A by the expression
r RMS = 0.836*A1/3+0.57 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.