Can anyone help me in doing MECP calculation using ORCA. What should be an ideal starting point for this type of calculation? I am expecting a crossing between S1 and T1 states. The optimisation and frequency calculation for GS and those two excited states have been completed.
The related keywords in ORCA is ! CI-Opt. You can refer to page 187 of ORCA 5.0.1 Manual. You may also need SF-TDDFT to give more accurate description of the electronic structure of crossing point. However, although TD-DFT is convenient for MECP locating, the best choice is to run multireference calculations using other programs like Molpro or Molcas.
By the way, the KST48 program (https://github.com/RimoAccelerator/KST48) is a universal interface to Gaussian, ORCA and BAGEL for MECP locating between any arbitrary two states with gradients available.
Dear Yumiao, Another doubt I was about to ask is, what should be my input geometry for this case? Is it the ground state optimised geometry or the S1 state optimised geometry? As I said, i m expecting a crossing between S1 and T1 states.
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