Spin-orbit effects calculated by two-component coupled-cluster methods: test calculations on AuH, Au2, TlH and Tl2

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 Kramersʹ 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.