Tip

All input files can be downloaded: Files.

mecp

Options
Theoretical Background
- MECP Between Ground and Excited States
- Diabatic MECP (dMECP)
Input Examples
- Example: MECP between the Singlet and Triplet States of C₆H₅⁺
- Example: Search Transition State of Iodine Atom Transfer Reaction

This keyword defines the arguments for searching minimum energy crossing point (MECP) and using diabatic MECP (dMECP) method to search transition states.

For standard MECP search, you can use the mecp keyword in task...end block. For using dMECP to search transition states, you can use the dmecp keyword in task...end block.

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.

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 

Hint

To use dMECP, you must use the dmecp keyword in task...end block instead of mecp. The obtained structures using dmecp and mecp are different. For transition states, we recommend to use ``dmecp`` instead of ``mecp`` to search the transition state.

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-2a.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
   dmecp b3lyp # Using dMECP method.
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.

You can also use mecp keyword instead of dmecp to to search the real MECP between the two diabatic states, the input files being as follows:

mecp-2b.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 # Search MECP between the two diabatic states.
end

The initial and optimized structures mecp-2*.inp, mecp-2a-mecp.xyz and mecp-2b-mecp.xyz are shown below:

Interestingly, both mecp-2a-mecp.xyz and mecp-2b-mecp.xyz have a single imaginary frequency, indicating that they are both close to transition states. However, the transition state obtained by dmecp is more stable than that obtained by mecp. For mecp-2b-mecp.xyz, we find that the energy difference between the reactant and product diabatic states is about 0.000008 Hartree, which is the definition of MECP. For transition states, we recommend to use ``dmecp`` instead of ``mecp`` to search the transition state.

mecp

Options 

Theoretical Background 

MECP Between Ground and Excited States 

Diabatic MECP (dMECP)

Input Examples 

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

Example: Search Transition State of Iodine Atom Transfer Reaction 

mecp

Options

Theoretical Background

MECP Between Ground and Excited States

Diabatic MECP (dMECP)

Input Examples

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

Example: Search Transition State of Iodine Atom Transfer Reaction

Options 

Theoretical Background 

MECP Between Ground and Excited States 

Input Examples 

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

Example: Search Transition State of Iodine Atom Transfer Reaction 