43.4 Recomendations

It is recommended to

• use the MASS keyword within the geometry specification in oder to rotate the molecule into standard orientation. This is necessary for a full exploitation of symmetry within the generation of the potential energy surface.
• split up the calculations in three parts: (1) Do a very accurate geometry optimization and bring the optimized geometry into standard orientation. Start a second calculation based on the optimized geometry and do (2) a harmonic frequency calculation which should be stored in the restart-file (.wfu). Make a backup of the restart-file. (3) Start a SURF, VSCF and VCI calculation from the restart-file (.wfu) and dump the potential energy surfaces to an external file. Later on you can restart your VSCF and VCI calculations from this external file without recomputing the surface.
• use a 1D: CCSD(T)/vtz, 2D: MP4(SDQ)/vtz and 3D: MP2/vtz multi-level scheme in combination with symmetry and a parallelized MOLPRO version in order to speed up the calculations.
• reduce the quality of the normal coordinates. If you do not require extremely high accuracy, it is sufficient to compute the normal coordinates (i.e. the harmonic frequencies) at the MP2 level rather than the CCSD(T) level. This saves a lot of computer time and the deviations are usually not larger than 1 or 2 wavenumbers.