Mixed Cartesian and Z-matrix coordinates

The key atoms can also be used to supply coordinates in a format that gives the values for the cartesian coordinates and the connection matrix, which defines a Z-matrix.

ATOMS ZCart
 {N} Atom Coords {F=Fragment}
 ...
End

ZCart

Signals this particular format for the coordinates

Coords

As for Z-matrix input: three integers and three real values. The integers are the connection numbers that define the Z-matrix structure, but the reals are the Cartesian coordinates.

With ZCart input, the z-matrix is internally generated from the Cartesian coordinates and the connection numbers.

This feature is convenient when for instance Cartesian coordinates are easily available but you want to run a Geometry Optimization in internal coordinates, for which a Z-matrix structure is required.

The zcart option comes in handy also to satisfy symmetry-related orientation requirements when you basically wish to use Z-matrix coordinates.

With zcart input the program defines the type of coordinates in the input file as Cartesian. This is significant in Geometry Optimizations, where the optimization variables are by default taken as the input coordinate type.