MESX is a local minimum on the crossing seam between two adiabatic PESs of different spin or space symmetry. MECI is a local minimum on the conical intersection between two adiabatic PESs of the same spin and space symmetry. An approximate geometry of MESX or MECI can be obtained as a local minimum on the following penalty function.

In this equation, *E*^{X} and *E*^{Y} are the adiabatic PESs of the two target states, **Q** is the coordinates, and *α* is a parameter (in default 30 kJ/mol). An example of input for an S_{0}/T_{1} MESX estimation is:

` # MIN/UB3LYP/6-31+G** 0 1 C 0.000000000000 -0.000000000000 -0.530465924142 O -0.000000000000 0.000000000000 0.685412425504 H 0.000000000000 0.942437931730 -1.129067750681 H -0.000000000000 -0.942437931730 -1.129067750681 Options ModelF Second Input UB3LYP/6-31+G** 0 3 END MaxStepSize=0.1 Stable=Opt `

The ** ModelF** option is used together with the

- Gaussian 03/09/16
- Molpro
- GAMESS

Updated At:Aug. 4, 2020, 2:02 p.m.