9.8 Low-scaling MP2 and MP2-F12 calculations with a hybrid OSV-PNO approximation

For a PNO-MP2 calculation with default thresholds and standard basis sets the pnoccsd program does not require any special input apart from $freeze and $maxcor and the input needed for the Hartree-Fock calculation. It is, however, recommended to specify the PNO truncation threshold in the data group $pnoccsd. For further details see Secs. 12 and 25.2.25.

Running pnoccsd parallel

The MP2 part of the pnoccsd program is parallelized with OMP for shared-memory and with MPI for distributed memory architectures. Important for the performance of the parallel pnoccsd calculations are the settings for the core memory usage and for the directories where large integral and scratch files are stored.

The keyword $maxcor defines for pnoccsd (as for ricc2 and ccsdf12) the core memory usage. Note, however, that $maxcor defines only the memory controlled by the electronic structure code. Additional memory can be allocated by the math and MPI libraries linked into the program and by the operating and I/O systems. It is therefore recommended to set $maxcor not higher than to 75% of the physical core memory that is available for the calculation.

For MPI parallel pnoccsd calculations it is very important to set with the $tmpdir keyword the path to directories in fast local file systems to avoid that large integral and scratch files are stored in the NFS file system where the calculation is started.