While the TS is defined as a first order saddle point on the potential energy surface (PES) of the molecule, no such rules exist for a CI. There is no valuable meaning you would obtain from the Hessian calculation of the CI.

In my opinion, if you need a path between FC and CI, you can do it one of two ways: either by Linear Interpolation in Internal Coordinates (LIIC) or by minimum energy path (MEP). For the first method, you will have to generate geometries depending on how different the FC is with respect to the CI and then calculate single point energies for the generated geometries. For MEP, you need to examine the FC and CI geometries closely to identify which coordinate is changing the most and then move along that coordinate in small steps and carry out constrained geometry optimization. The state on which you need to optimize the geometry depends on which state the FC geometry is present on, after excitation.

While MEP is more accurate, LIIC is computationally very cheap.

Can you please let me know how you can calculate linear interpolation between two geometries? and which software do you use to calculate it?

Dear Nita Ghosh, there are a few ways to do the interpolation. The simplest way is to use the linear interpolation in cartesian coordinates, meaning that you start with coordinates of initial geometries and then keep on adding the difference (Initial - Final ) till you get the final geometry. Other ways are to do the linear interpolation in internal coordinates or using the distance matrix. It would be best if you searched for the geodesic interpolation. There are many other methods. I found that there is one module for interpolation in Columbus software. Try finding interpolation codes in Github.

Regards,

Dilawar