The one-and two-electron spin-orbit integrals over the BP Hamiltonian can be precomputed and stored on disk using the command
LSINT[,X][,Y][,Z]
X, Y, and Z specify the components to be computed. If none of these is given, all three are evaluated. The advantage of precomputing the integrals is that they can then be used in any number of subsequent SO calculations, but this may require a large amount of disk space (note that there are 6 times as many integrals as in an energy calculation). If the LSINT card is not given, the integrals are recomputed for one component at a time whenever needed, and destroyed at the end of the SO calculation. This reduces the disk space by a factor of 3, but may be expensive in terms of CPU if several SO calculations (e.g., for MCSCF and MRCI wavefunctions) are carried out.
The input for spin-orbit ECPs is described in section 11. Of course, in ECP-LS calculations the LSINT card is not needed.