When running a transition state search or geometry optimisation in orca, some of the quantites like forces in cartesian coordinates are only written to the *.opt file that is not mentioned in the manual. Here is a description of the file contents and formats.

The file is set up in sections where the labels of which are prefixed with the *$* sign:

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$orca_opt_file $trust $epredict $ediffsc $ctyp $coordinates $TSSearch $redundant_internals $energies $gradients $redundant_coords $redundant_gradients $hessian_approx $bmatrix |

The contents of the “$coordinates” section is

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$coordinates numframes numcoordinates cycle1_atom0_x cycle1_atom0_y cycle1_atom0_z cycle1_atom1_x ... ... cycle1_atomN_z cycle2_atom0_x cycle2_atom0_y ... |

In orca, the geometry optimisation cycles start to count from 1, while atoms count from 0. The coordinates are given in Angstrom. *Numframes* gives the number of geometries in this file, *numcoordinates* is the number of cartesian coordinate components, i.e. three times the atom count.

The section “$energies” is given as

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$energies numframes final_single_point_energy_cycle_0 final_single_point_energy_cycle_1 ... |

All energies are given in Hartree.

Gradient information is given in the “$gradient” section as follows

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$gradients numframes numcomponents cycle1_atom0_x cycle1_atom0_y cycle1_atom0_z cycle1_atom1_x ... ... cycle1_atomN_z cycle2_atom0_x cycle2_atom0_y ... |

This follows the logic of the “$coordinates” section. All gradients are given in Hartree / bohr. Note that this is the actual gradient, so the force is the negative value of this (yes, cp2k, I am looking at you).

The other sections include current TS mode components ($TSSearch), definition of redundant internal coordinates ($redundant_internals), coordinates for all cycles in internal coordinates ($redundant_coords), gradients for all cycles in internal coordinates ($redundant_gradients), the current (!) approximation of the hessian matrix ($hessian_approx) and the Wilson B matrix.