Tip

All input files can be downloaded: Files.

mecp

This keyword defines the arguments for searching minimum energy crossing point (MECP).

Hint

When using this keyword, in scf...end block, you must ALWAYS set type u.

Options

num_steps

Value	An integer
Default	200

The maximum number of MECP search.

energy_cov

Value	A real number
Default	1.E-5

The energy difference convergence threshold. When energy_cov, grad_cov, and dr_cov are all met, the MECP will be set to be converged.

grad_cov

Value	A real number
Default	1.E-3

The gradient convergence threshold. When energy_cov, grad_cov, and dr_cov are all met, the MECP will be set to be converged.

dr_cov

Value	A real number
Default	1.E-3

The displacement convergence threshold. When energy_cov, grad_cov, and dr_cov are all met, the MECP will be set to be converged.

orb1

This defines the electronic configuration of state 1. The format is:

orb num_electrons spin_multiplicity  alpha_MO_indices : beta_MO_indices

There can be arbitrary number of orb1, but all orbitals must be included once and only once. They are the same as orb in scfguess. See scfguess.

orb2

This defines the electronic configuration of state 2. The format is:

orb num_electrons spin_multiplicity  alpha_MO_indices : beta_MO_indices

There can be arbitrary number of orb2, but all orbitals must be included once and only once. They are the same as orb in scfguess. See scfguess.

frag1

This defines the fragmentation of state 1. The format is:

frag num_electrons spin_multiplicity atom_range

There can be arbitrary number of frag1, but all atoms must be included once and only once. They are the same as frag in scfguess. See scfguess.

frag2

This defines the fragmentation of state 1. The format is:

frag num_electrons spin_multiplicity atom_range

There can be arbitrary number of frag2, but all atoms must be included once and only once. They are the same as frag in scfguess. See scfguess.

Hint

When frag1/2 and orb1/2 are defined simultaneously, orb1/2 will be considered in priority.

Theoretical Background

MECP Between Ground and Excited States

The minimum energy crossing point (MECP) is a point where two potential energy surfaces (PES) of different electronic states cross each other. In the context of quantum chemistry, it is often used to describe the transition between a ground state and an excited state. The MECP can be found by optimizing the geometry of the system while constraining the energy difference between the two states to be zero.

Diabatic MECP (dMECP)

Diabatic MECP (dMECP) is a method used to find the transition state for a AB+C=A+BC reaction. It uses the MECP search algorithm to find the minimum energy crossing point between the reactant and product diabatic states.

Input Examples

Example: MECP between the Singlet and Triplet States of C₆H₅⁺

In this example, we will search the MECP between the singlet and triplet states of C₆H₅⁺. The input file is as follows:

mecp-1.inp

basis
   def2-svp
end

mol
 C                  0.00000000    1.39661300    0.00000000
 C                  1.20950300    0.69830700    0.00000000
 C                  1.20950300   -0.69830700    0.00000000
 C                  0.00000000   -1.39661300    0.00000000
 C                 -1.20950300   -0.69830700    0.00000000
 C                 -1.20950300    0.69830700    0.00000000
 H                  2.15088200    1.24181200    0.00000000
 H                  2.15088200   -1.24181200    0.00000000
 H                  0.00000000   -2.48362500    0.00000000
 H                 -2.15088200   -1.24181200    0.00000000
 H                 -2.15088200    1.24181200    0.00000000
end

mecp
    num_steps     200   # The number of search steps.
    energy_cov    1.E-5 # The energy difference convergence threshold.
    orb1 40 1 1-109 : 1-109
    orb2 40 3 1-109 : 1-109
end

scf
   type u
   charge  +1
   spin2p1  1
end

task
   mecp b3lyp
end

In mecp...end block, we define the two electronic configurations of the singlet and triplet states using orb1 and orb2. The num_steps is set to 200, and the energy convergence threshold is set to 1.E-5.

The format of orb1 and orb2 is the same as that in scfguess. For example, orb1 40 1 1-109 : 1-109 means that the first state has 40 electrons, a spin multiplicity of 1, and the alpha and beta MOs are from indices 1 to 109. The same applies to orb2.

The singlet-triplet MECP is saved to mecp-1-mecp.xyz. The optimized process is saved in mecp-1-mecp-traj.xyz, which can be shown using Qbics-MolStar:

Example: Search Transition State of Iodine Atom Transfer Reaction

In this section, we want to search the transition state of the following reaction:

This is a AB+C=A+BC type reaction. The largest advantage of dMECP algorithm is that the transition state is searched using energy coordinate instead of geometry coordinate, so it is not too sensitive to the initial guess.

The input file is as follows. Note that for iodine, we have used pseudopotential with pseudopotential keyword, def2-ecp.

mecp-2.inp

basis
    def2-svp
end

pseudopotential
    def2-ecp
end

scf
    type     u
    charge   0
    spin2p1  2
end

mol
    C    -0.29500    -0.00002    -0.00017
    C    1.10016    -0.00002    -0.00017
    C    1.79770    1.20773    -0.00017
    C    1.10004    2.41624    -0.00137
    C    -0.29478    2.41616    -0.00185
    C    -0.99238    1.20795    -0.00085
    H    -0.84476    -0.95234    0.00028
    H    1.64967    -0.95253    0.00114
    I    3.87770    1.20788    0.00103
    H    1.65024    3.36838    -0.00143
    H    -0.84490    3.36844    -0.00280
    H    -2.09199    1.20814    -0.00103
    Si    6.95770    1.20810    0.00280
    C    7.59353    0.30695    1.56221
    C    7.59533    0.30859    -1.55681
    C    7.59423    3.00892    0.00412
    H    7.23873    -0.70024    -1.55754
    H    7.23913    0.81342    -2.43040
    H    8.66533    0.30866    -1.55619
    H    8.66353    0.30703    1.56282
    H    7.23632    0.81087    2.43592
    H    7.23694    -0.70188    1.56147
    H    8.66423    3.00899    0.00563
    H    7.23876    3.51349    -0.86992
    H    7.23629    3.51309    0.87738
end

mecp
    num_steps   200
    energy_cov  1.E-5
    frag1   0 1 1-12
    frag1   0 2 13-25
    frag2   0 2 1-8 10-12
    frag2   0 1 9 13-25
end

task
   mecp b3lyp
end

The structure given in mol...end is shown below:

We can see that the initial structure is a complex that two reactants are simply putting together. Now, according to the reactant and product species, we can define frag1 and frag2 in mecp...end option block. The definitions are shown below, which is self-explanatory and in line with chemical intuition:

In dMECP method, only a single geometry is needed. Of course, the closer it approaches the transition state, the better.

The optimized structure mecp-2-mecp.xyz is shown below: