Approximate structures of MESX and MECI can be explored automatically by the ADDF method. ADDF is applied to the following model function F(ξ) called seam model function (SMF), which consists of a mean energy term for the two target PESs, EX(Q) and EY(Q), and a penalty function for their energy gap,
where Q is the atomic coordinates and α is a constant parameter (in default, α = 30 kJ/mol). Typically, α is set to ∼1/10 of the vertical excitation energy in the target system, and in general results are not very sensitive to the value of α. Minimization of F(Q) gives a geometry in which both the mean energy and the energy gap are small. Hence, local minima on F(Q) can be good guesses of MESXs and MECIs.
An example of input for an automated exploration of approximate S0/T1 MESX structures is:
C -0.450261277786 0.001229457926 -0.553082461345
O 0.054687651811 -0.003728650367 0.666230115529
H 0.197675109173 0.761833114909 -1.107795147264
H 0.197898516798 -0.759333922472 -1.108541506919
The ModelF option is used together with the Second Input … END option. Lines between Second Input and END specify the computation method and [spin charge] 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. When ModelF option is specified, EQOnly is also applied. In the xxx_EQ_list.log file of this calculation, many local minimum structures on the model function SMF are obtained. Structures obtained by this calculation are just approximate MESX or MECI structures, and therefore need to be reoptimized with OptX.
Note: the initial structure in the above input is optimized local minimum on the model function SMF.