I'm trying to compute potential energy surface of S4 molecule (in MOLPRO 2015) along S2-S2 bond (it has a trapezoidal shape) from equilibrium to the dissociation threshold, all the other coordinates are fixed. S4 is a singlet (0 unpaired electrons) in the ground state but a quintuplet once it's dissociated (2 independent S2 molecules in the ground state, each has 2 unpaired electrons). I tried to do CCSD(t) calculations with spin 0 and 2. With spin=0 I get the right energy at the equilibrium but too high energy at the dissociation limit and it's vice versa for spin=2. I need a single surface to solve the Schrodinger equation in so I was thinking about just taking minimum between the two surfaces but I'm not sure if it's a correct way to do it.
This problem doesn't seem to be that rare so I think there should be a well known way to tackle it. I think MRCI can use a mix of the two states, adjusting coefficients in the process to make the transition smooth, but I'm not sure how to properly do it.