Note

This website is presently under development. The primary website for MOPAC is still http://openmopac.net until the majority of its contents have been migrated here.

INT

Regardless of what type of coordinates are used in the data set, INT will force all the coordinates to be internal coordinates. Atom 1 is, by definition, always defined in Cartesian coordinates.

Notes

• If  INT is used, the optimization flags are not changed. Therefore, before running a system with INT all the optimization flags should be set to “1”, except for atoms 1, 2, and 3. For atom 1, all optimization flags should be set to “0”, for atom 2, the second and third flags should be “0”, and for atom 3 the third flag should be “0”.

• Be very careful if some atoms have optimization flags set to “0”.  If the geometry supplied is in Cartesian coordinates, and an atom, say atom 10, has all three optimization flags set to “0”, then that means “Do not change the Cartesian coordinates of atom 10.”   If that atom is converted to internal coordinates, the optimization flags will still be zero, but now the definition changes to “Do not change the bond length, angle, and dihedral of atom 10, with reference to the atoms used for the connectivity.”  Now atom 10 will move if the atom it is attached to moves. This is fundamentally different from the Cartesian coordinate definition.

• Any dummy atoms are automatically deleted if INT is present. This is because the first step in running INT is to convert the system to Cartesian coordinates.  This involved deleting any dummy atoms.  The next step involves conversion from Cartesian to internal coordinates.

•  INT is a rapid and efficient way of deleting atoms from a molecule that is defined using internal coordinates.  Re-label the atoms to be deleted as dummy atoms, and run INT with 0SCF.

See also XYZ.