The ground state of NO splits into two spin orbit states (2pi1/2 and 2pi3/2). Does anybody know how to compute the energies of these states using either Gaussian or Molpro?
Molpro uses the state interacting method to calculate spin-orbit coupling. First you calculate the spin-free MRCI or CASSCF wave function, and then you use the CI code to calculate SO matrix elements between these and diagonalize the result. For extensive details see the manual.