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, EX and EY 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 S0/T1 MESX estimation is:
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
The ModelF option is used together with the Second Input … END option. Lines between Second Input and END specify the computation method (UB3LYP/6-31+G**), charge (0) and spin multiplicity (3) of the second state. Use of MaxStepSize=x (x ≤ 0.1) and Stable=Opt options is recommended. When the MO GUESS option is used, two MO files, i.e., mmm.chk and mmm.chk_1, are required for the first and second states, respectively. Contrary to OptX, a relatively large energy gap remains between the two target states; the structure must be optimized again by OptX before using the structure in quantitative discussions.