Spin-orbit effects calculated by two-component coupled-cluster methods: Test calculations on AuH, Au ₂ , TlH and Tl ₂

We have modified the MOLFDIR code by Visscher and coworkers to use one-electron integrals of relativistic effective core potentials with spin-orbit operators and two-component molecular spinors for polyatomic molecules. The present methods incorporate spin-orbit interactions from the Hartree-Fock step through the use of two-component molecular spinors confirming Kramers' degeneracy and are particularly useful to estimate spin-orbit effects. Test calculations for the ground states of AuH, Au₂, TlH and Tl₂ are performed to show that spin-orbit effects vary depending upon the level of theory employed.