The parallel processing mode can be changed with this keyword.
HXYZ Parallelization of frequency analysis over coordinates. This is the default.
HSCF Parallelization of frequency analysis over the SCF.
GRAM Store inverse Coulomb matrix in RAM.
In a parallel single-point calculation, the work is distributed inside the SCF. That is, the integral calculation and linear algebra are parallelized and therefore the various CPUs all work on the same SCF calculation. We call this parallelization over the SCF. For the calculation of second derivatives (see 4.8.1 and 4.6.5), a different parallel assignment is made. Here each CPU works alone on a distinct SCF cycle. We call this parallelization over the coordinates. This is the default for the numerical second derivative calculation. But such parallelization also means that each process uses a complete set of matrices and files, just as a calculation on a single CPU would. As a result, performance problems can occur on some computational architectures, mainly because of swapping and/or intensive I/O. In those cases, use of the option HSCF will change the parallelization scheme to the one which is used for a single-point SCF calculation, i.e. all CPUs work on the same SCF. That approach is less efficient and should be used only if necessary.

The GRAM option of the PARALLEL keyword triggers the in-core storage of the inverse Coulomb matrix [25] in frequency calculations. This increases the RAM demand of the calculation but reduces I/O operations. We recommend this option for parallel runs of smaller systems with less than 500 atoms. This option has no effect when the option HSCF is also active.