TITLE NH2 - open-shell MP2 optimisation and FCM SYMMETRY CNV 2 END MULTIPLICITY 2 BASIS DZP ATOMS NITROGEN 7.0 0.0 0.0 0.0 HYDROGEN 1.0 1.5181889 0.0000000 -1.1813339 END ROHF OPTIMISE MP2 START NPOINTS 2 FORCE START FINISHThis performs a geometry optimisation, and then a numerical FCM calculation. The keyword NPOINTS 2 chooses a central-difference differentiation technique. ie.
is a forward-difference method, and
is a central difference method. Central difference methods are more accurate, but take twice as long as forward-difference calculations. To reduce the expense the program uses translational invariance and symmetry to make as few displacements of the atoms as possible. To reduce numerical errors the program automatically takes slightly higher convergence criteria and internal accuracies when calculation FCMs by finite difference. The dipole moment derivatives are also produced by the calculation, and hence the infra red intensities are calculated, as well as the harmonic frequencies.
The information i.e. the frequencies, normal modes and intensities, is output in the same format as for the analytic cases discussed previously.