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39.2 Calculation of SO integrals

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][,ONECENTER][;TWOINT,twoint;][;PREFAC,prefac;]

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 computed whenever needed. The keyword ONECENTER activates the one-center approximation for one- and two- electron spin-orbit integrals. This can reduce drastically the computing time for large molecules. TWOINT and PREFAC can be used to control the accuracy of spin-orbit integrals. These thresholds are similar to TWOINT and PREFAC for standard integrals. The default value for PREFAC is TWOINT/100, and the default value for TWOINT is $10^{-7}$. In the case when no integrals are precomputed, these thresholds can be specified as options for HLSMAT or TRANLS cards, see below.

The input for spin-orbit ECPs is described in section 12. Of course, in ECP-LS calculations the LSINT card is not needed.

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