This keyword assigns constant Cartesian and internal structure parameters.
Options: None
The body of CONSTANTS lists all atoms and internal coordinates which are kept frozen during the geometry optimization. The atomic or element symbol with the string XYZ is used to freeze the atom or group of atoms in redundant coordinates. The string X will only freeze the x coordinate of the atom, the string XY the x and y coordinates, etc.

For internal coordinate inputs, freezing of a particular degree of freedom is done by giving the symbolic coordinate string of the Z-matrix and its assigned (frozen) value (real number). The maximum length of the value string is 15 characters. Please note that structure parameters that are explicitly defined inside the Z-matrix are always treated as CONSTANTS in optimizations. In the case of a MIXED input, these two syntax forms can be mixed (see Figure 4 and the corresponding input example), for example in the form

 H    XYZ
 A  120.0

Here, the first line freezes all hydrogen atoms at their positions. In the second line the internal coordinate A is frozen to 120. To avoid inconsistencies in the definition of mixed constraints, it is advisable to freeze three (linearly independent) atoms of the Cartesian part of the mixed input. These atoms may also be dummy atoms. In this way, a global reference coordinate system is defined (see example [*] on page [*] of the tutorial). A similar strategy should be used for mixed constraints on internal coordinates, as the following input example shows.

 Geometry Z-Matrix
 Fe   X1   1.0
 X2   Fe   0.5    X1  90.0
 X3   Fe   0.5    X1  90.0   X2  180.0
 X4   Fe   rFeCp  X1  90.0   X3  180.0
 X5   Fe   rFeCp  X1  90.0   X2  180.0
 C1   X4   rCX    Fe  90.0   X1  0.0
 C2   X4   rCX    C1  72.0   Fe  90.0
 C3   X4   rCX    C2  72.0   Fe  90.0
 C4   X4   rCX    C3  72.0   Fe  90.0
 C5   X4   rCX    C4  72.0   Fe  90.0
 C6   X5   rCX    Fe  90.0   X1  0.0
 C7   X5   rCX    C6  72.0   Fe  90.0
 C8   X5   rCX    C7  72.0   Fe  90.0
 C9   X5   rCX    C8  72.0   Fe  90.0
 C10  X5   rCX    C9  72.0   Fe  90.0
 H1   C1   rCH    X4  alpha  Fe  180.0
 H2   C2   rCH    X4  alpha  Fe  180.0
 H3   C3   rCH    X4  alpha  Fe  180.0
 H4   C4   rCH    X4  alpha  Fe  180.0
 H5   C5   rCH    X4  alpha  Fe  180.0
 H6   C6   rCH    X5  alpha  Fe  180.0
 H7   C7   rCH    X5  alpha  Fe  180.0
 H8   C8   rCH    X5  alpha  Fe  180.0
 H9   C9   rCH    X5  alpha  Fe  180.0
 H10  C10  rCH    X5  alpha  Fe  180.0
 rFeCP   1.19723
 rCH     1.09414
 alpha 162.12553
 rCX 1.21719
 X1  XYZ
 X2  XYZ
 X3  XYZ
 Fe  XYZ

Furthermore it is advisable, though not mandatory, that mixed constraints be defined to match the mixed input, i.e. Cartesian constraints for atoms that are defined by Cartesian coordinates and internal constraints for the bonds and angles that are defined in the Z-matrix part of a mixed input. Examples [*] and [*] on page [*] and [*] of the tutorial demonstrate the use of mixed constraints in combination with equivalent and constant coordinate definitions.