It is recommended to
- use the ORIENT,MASS keyword in order to rotate the molecule into standard
orientation. This is necessary for a full exploitation of symmetry within the generation of the potential
- use the MASS,ISO keyword to use the most available isotopes.
- use multi-level schemes in combination with symmetry and
a parallelized MOLPRO version (MPPX) in order to speed up the calculations. Explicitly correlated methods
are preferable over conventional approaches.
- 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 DFT 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 in the subsequent VSCF and VCI calculations.