Q. What are the reference state and the active space for the perturbation selection in the general-R?

 

A. In the default general-R calculations, SDCI solutions are used for the reference states of the perturbation selection. This reference SDCI is performed with a small active space and diagonalized by in-core algorithm.

The size of the active space in the SD-CI is automatically determined, according to the memory size and the orbital energies. User can specify the active space with "RefCIWindow" option. However, when the estimated SDCI dimension for each symmetry exceeds 2000, the active space is reduced regardless of the memory size. If users want to specify this small active space for large reference SDCI dimension, "Namelist" option should be used as follows.

 

# SAC-CI(…, Namelist)

  

&ghpres mxdiag=5000, &End

 

where the maximum dimension of 5000 is specified.