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
Constants 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 X1 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 # VARIABLES rFeCP 1.19723 rCH 1.09414 alpha 162.12553 # CONSTANTS 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.