44.4 Approximations used in calculating spin-orbit integrals and matrix elements

Recently, more sophisticated approximations were introduced to simplify spin-orbit calculations for larger molecules. These are controlled by specifying the spin-orbit operator type lsop as follows (we omit suffixes X, Y, Z which specify the component):

Standard spin-orbit calculations.
The one-center approximation is used for one- and two-electron spin-orbit integrals.
The effective Fock-matrix approximation is used for the internal part too.
The one-center approximation is used for one- and two-electron spin-orbit integrals, and the effective Fock-matrix approximation for the internal part.
Effective core potentials are used for all atoms at which they are defined; contributions of all other atoms are neglected (see below).

In case that the effective Fock matrix is used for all contributions, and no spin-orbit integrals are pre-calculated and stored on disk (i.e., the LSINT command is not given), the Fock matrices are evaluated in direct mode and no integrals are stored on disk. When this is combined with the one-center approximation (AMFI), the computing and I/O times are drastically reduced, and this makes spin-orbit calculations quite fast even for larger molecules.

Also, the treatment of ECP-type of spin-orbit interaction has been changed and now allows for treating both ECP and non-ECP atoms in one calculation. Thus, in molecules containing both heavy and light atoms, the heavy atoms can be described using ECPs and the light atoms using all-electron basis sets. If the operator type is LS, ALS, FLS, or AFLS, then for the atoms having an ECP spin-orbit operator defined in the basis input the ECP operator is used, while the full BP-operator is used for all other atoms (couplings are neglected). Both one-center and AMFI approximations can be used in this case. If, on the other hand, one specifies the operator type as ECPLS, then the behavior is the same as in the previous versions, i.e., only the ECP contributions are considered and the contributions from all other atoms are neglected.

molpro@molpro.net 2019-06-16